FROM  THR  AUTHOR'S  COLLECTION. 


COPYRIGHT  1895,  BY  HOWARD  HART. 


...THB... 


CHEMISTRY  OF  POTTERY 


BY 


SUPERINTENDENT  OF  THE  MOSAIC  TIL.E  COMPANY,  ZANESVH,I,E,  O., 

FORMERLY  SUPERINTENDENT  OF  ROOKWOOD  POTTERY, 

CHEMIST  OF  THE  AMERICAN  ENCAUSTIC 

TILING  COMPANY,  ETC. 


KASTON,  PA.: 

CHEMICAL   PUBLISHING   CO. 
1895. 


DEDICATED 

TO   MY 

ASSOCIATE   IN   CERAMIC  WORK, 

HERMAN  C.    MUEUvER, 

SCULPTOR. 


CONTENTS. 


Page. 
Chapter  I.          Analysis  of  Pottery  Materials  and  Products-  •       i 

"        II.  Physical  and  Empirical  Tests ^5 

"        III.  Pyrometry 26 

"        IV.  Classification  of  Ceramics 41 

V.  Pottery  Glazes 48 

VI.  Red  Ware 58 

VII.  Rockingham  and  Yellow  Ware 66 

"        VIII.  Stoneware   77 

"        IX.  Raw  Materials  of  White-Ware  Bodies 93 

"        X.  White  Granite  and  Cream-Colored  Ware 117 

"        XI.  Majolica  and  Enameled  Tile 127 

"        XII.  White  Enameled  Brick 139 

XIII.  Floor-Tile  and  Terra-Cotta 149 

"        XIV.  Refractory  Materials 158 

"        XV.     Burning  the  Ware 175 


38S233 


LIST  OF  ILLUSTRATIONS. 


Plate  i.  From   the  author's  collection Frontispiece. 

"     2.  Clay  Bank",  Muskingum  Co.,  Ohio to  face  p.  97. 


Page. 

Initial i 

"       - 15 

Vicat's  Needle 19 

Gas  Trial  Kiln 23 

Initial 26 

A  vSeger  Cone  in  Natural  Size 36 

Initial 41 

Tailpiece 47 

Initial 48 

Tailpiece 57 

Initial 58 

"       66 

"       77 

11       93 

"       JI7 

;*    127 

Tailpiece 138 

Initial 139 

"       H9 

"       158 

11       175 

Kiln  Barometer  or  Draught  Meter 178 


PREFACE. 


The  pottery  industries  of  Kngland  and  America  have 
afforded  chemists  little  opportunity  for  systematic  work, 
so  that  they  have  remained  largely  on  an  empirical  basis 
and  have  supplied  nothing  of  moment  to  chemical  tech- 
nology. 

With  the  exception  of  the  excellent  treatises  on  porce- 
lain, but  little  work  relating  strictly  to  pottery  has  been 
published  in  France. 

The  best  of  such  work,  accessible  to  chemists,  is  that 
of  German  ceramists  and  has  appeared  in  that  excellent 
technical  periodical,  "Die  Thonindustrie-Zeitung," 
founded  by  the  late  Professor  Seger,  sometime  chemist 
of  the  Royal  Porcelain  Factory  at  Charlottenburg,  near 
Berlin. 

Most  of  the  technical  publications  on  pottery,  in  book 
form,  dwell  on  the  mechanical  technology  of  the  clay 
industries,  the  chemical  parts  consisting  merely  of  re- 
ceipts, claiming  a  practical  origin. 

Such  receipts  are  almost  altogether  without  value,  as 
the  materials  specified  are  not  characterized  by  accom- 
panying analyses  and  the  temperatures  to  which  the 
products  are  to  be  subjected  are  not  given,  or  not 
determined  in  a  way  that  they  can  be  fixed  with  certainty. 


VI  PREFACE. 

While  to  many  of  our  chemists,  engaged  in  develop- 
ing the  natural  resources  of  the  country  the  clays  found 
on  every  side  have  seemed  to  offer  a  fruitful  field  for 
investigation,  the  information  at  command  concerning 
the  chemical  needs  of  the  potter  has  been  so  meager, 
that  their  efforts  have  been  practically  abortive.  The 
thousands  of  analyses  published  are  mostly  worthless, 
because  they  do  not  go  far  enough  or  because  unaccom- 
panied by  essential  physical  tests  and  practical  trials. 

It  is  hoped  that  this  little  treatise  will  supply  informa- 
tion that  will  turn  future  labors  in  this  channel  to  good 
account. 

The  writer  is  well  aware  that  in  confining  the  subject 
matter  to  the  results  of  his  personal  experience  the 
shortcomings  of  the  treatise  are  numerous  and  manifest ; 
but  he  believes  that  as  such  it  is  a  more  direct  expres- 
sion of  the  practical  needs  of  the  working  potter,  and 
therefore  of  more  immediate  value,  than  a  compilation 
of  the  published  work  of  European  chemists  on  their 
ceramic  industries. 

In  a  treatise  of  this  kind,  only  the  essentials,  pottery 
bodies  and  glazes,  could  be  considered.  The  colors  and 
operations  of  decorating,  though  interesting  objects  of 
chemical  study,  would  have  led  too  far  from  the  main 
purpose  of  the  book. 


CHAPTER  I. 

ANALYSIS  OF   POTTERY    MATERIALS    AND 
PRODUCTS. 


T  IS  most  difficult  to  convey  to  the  public 
the  necessity  of  exercising  the  greatest 
care  in  taking  the  sample  of  a  clay  for  exam- 
ination. 

Chemists  and  engineers  will,  of  course,  appre- 
ciate this,  though  they  often  do  not  realize  that 
a  carelessly  taken  sample  is  absolutely  worth- 
less for  even  a  preliminary  test. 

The  usefulness  of  a  clay  for  a  given  purpose 
often  hinges  on  a  slight  point  of  plasticity  and 
shrinkage  or  color  at  a  certain  temperature, 
which  may  either  have  been  imparted  to  it,  or 
taken  away  from  it,  by  a  few  years  weathering  at  the 
exposed  outcrop  of  a  deposit,  or  by  the  infiltration,  or 
washing  out,  at  that  point,  of  small  amounts  of  iron,  lime, 
or  alkalis. 

The  first  rule  in  clay  examinations,  then,  is  that  a 
clay  deposit  which  does  not  warrant  the  expense  of 
thorough  sampling  does  not  justify  even  a  preliminary 
examination, — for  we  have  not,  as  in  iron-ore,  for  exam- 
ple, a  constituent  which  must  be  contained  in  at  least  a 
certain  amount,  and  which  any  exposure  is  not  likely  to 
* 


CtlEMISTRY   OF   POTTERY. 

materially  vary,  nor  are  there  constituents  which  must 
be  practically  absent. 

Every  constituent  of  a  clay  is  valuable.  It  is  the  work 
of  the  chemist  to  determine  to  what  use  it  can  be  put ; 
and,  considering  its  constituents,  to  what  conditions  of 
manufacture  it  must  be  subjected. 

His  first  question,  then,  is,  whether  there  be  enough 
like  the  sample  to  warrant  difficult  tests. 

If  the  clay  be  not  too  hard,  and  superimposed  strata 
too  difficult  to  work,  it  is  best  to  take  the  samples  over 
a  larger  area  by  systematic  borings,  using  a  one  and  one- 
half  to  two-inch  pod-auger,  welded  to  a  section  of  gas- 
pipe  for  the  purpose. 

If  the  overlying  strata  are  sandy  or  otherwise  liable, 
by  dropping  down  into  the  bore-hole,  to  vitiate  the  char- 
acter of  the  sample,  it  becomes  necessary  to  exclude 
them  by  driving  down  a  section  of  tubing  of  suitable 
width  to  the  top  of  the  clay  stratum  which  is  to  be  sam- 
pled. 

If  sampling  by  boring  be  not  practicable,  the  deposit 
must  be  opened  at  as  many  exposures  as  is  practicable 
and  laid  bare,  if  possible,  through  its  entire  depth,  well 
beyond  the  frost  line  or  other  surface  influence,  and  the 
sample  there  taken  down  the  whole  exposed  face  of  the 
vein. 

The  sampling  down  of  the  clay  obtained  by  these 
various  means  is  done  in  the  manner  familiar  to  all  accus- 
tomed to  handling  ores,  and  should  be  done  in  the  labor- 
atory, if  the  man  in  the  field  is  not  perfectly  familiar 
with  the  process. 


ANALYSIS   OF   MATERIALS   AND   PRODUCTS.  3 

In  preparing  a  sample  of  clay  for  chemical  analysis,  it 
is  important  to  bear  in  mind  what  constituents,  separa- 
ble by  mechanical  means,  the  clay  contains  which  are 
objectionable  in  the  industry  in  which  it  is  likely  to  be 
used,  and  to  reject,  by  a  suitable  treatment,  from  the 
analytical  sample,  those  constituents  that  will  not  enter 
into  the  ultimate  product,  though  it  is,  of  course,  neces- 
sary to  at  least  approximately  determine  their  character 
and  amount,  as  this  determines  the  future  mechanical 
purification  process  to  which  such  a  clay,  if  used,  would 
have  to  be  subjected. 

Thus  "lime-dogs"  and  pebbles  are  objectionable  in 
terra-cotta,  brick  and  red  ware  clays,  and  are  custom- 
arily removed,  if  present,  by  screening  the  dried  and 
crushed  clay  through  a  sieve  of  ten  to  twenty  meshes  to 
the  inch. 

Coarse  sand  and  particles  of  iron-pyrites  are  objection- 
able in  finer  terra-cotta,  floor  tile  and  yellow  ware,  and 
are  removed  by  disintegrating  the  clay  in  water,  passing 
the  "  slip"  through  a  sieve  of  sixty  meshes  to  the  inch, 
and  drying  the  purified  clay. 

In  clays  for  finer  ware,  flakes  of  mica,  in  addition  to 
the  last  mentioned  impurities,  are  objectionable,  and 
must  be  removed  by  passing  the  "slip"  through  a  looto 
1 20  mesh  sieve. 

It  may  seem  superfluous  to  say  that  the  chemical  anal- 
ysis of  a  clay  should  be  as  accurate  as  possible  ;  yet  the 
very  considerable  number  of  slovenly  analyses  published 
yearly  would  seem  to  make  it  necessary  to  insist  upon 
this  point. 


4  THK   CHEMISTRY  OF   POTTKRY. 

By  far  the  greater  number  of  technical  analysts  are 
engaged  in  making  single  determinations,  and  not  com- 
plete analyses  of  the  substances  with  which  they  have  to 
deal.  Clays,  in  which  all  the  constituents  are  of  equal 
importance,  seem,  therefore,  to  many  as  difficult  as  they 
certainly  are  tedious,  to  analyze,  and  the  determinations 
of  their  various  constituents  are  treated  as  if  they  were  a 
collection  of  individual  determinations,  the  incorrect 
footing  up  of  which  is  an  "arbitrary  impertinence," 
which  may  be  ignored. 

While  it  is  true  that  the  chemical  analysis  alone,  how- 
ever accurate,  is  insufficient  without  accompanying  phys- 
ical tests,  to  give  one  a  perfect  characterization  of  a  clay, 
its  value  is  sufficiently  great  to  warrant  the  most  careful 
work. 

Although  the  scope  of  this  book  makes  it  necessary  to 
refer  to  works  on  analytical  chemistry  for  the  details  of 
clay  analysis,  it  seems  desirable  to  point  to  special  con- 
siderations upon  which  text-books  do  not  sufficiently  insist. 

The  character  of  a  clay  depends  largely  upon  the 
mutual  proportions  of  alumina  and  silica,  so  that, 
although  these  are  generally  the  largest  constituents  of 
a  clay,  it  is  important  that  they  be  determined  with  par- 
ticular accuracy. 

In  spite  of  the  various  and  detailed  descriptions  of  the 
treatment  of  the  residue  of  the  acidified  fusion  of  a  clay 
with  alkali  carbonates,  it  seems  practically  impossible  to 
accurately  separate  silica  from  the  alumina  group,  and 
to  obtain  both  without  mutual  contamination.  The  wri- 
ter, therefore,  thinks  it  indispensable  to  finally  obtain  the 


ANALYSIS   OF    MATERIALS   AND   PRODUCTS.  5 

proportion  of  silica  by  difference,  smoking  off  that 
obtained  from  the  fusion  of  the  clay  with  alkali  carbon- 
ates, with  hydrofluoric  acid,  taking  cognizance  of  the 
residue  obtained  from  this  operation. 

The  alumina  and  ferric  oxid  precipitate  must  simi- 
larly be  obtained  by  difference,  when  the  silica,  which  it 
almost  invariably  contains,  has  been  separated  by  dis- 
solving the  weighed  precipitate  in  Mitscherlich's  mix- 
ture, or,  if  very  refractory,  by  fusion  in  acid  potassium 
sulfate,  the  silica,  after  weighing  and  volatilizing  as 
fluorid,  being,  of  course,  added  to  the  main  portion. 

Occasionally,  the  errors  due  to  alumina  in  the  silica 
and  silica  in  the  alumina  group  mutually  compensate 
each  other  within  the  limits  of  ordinary  analytical  accu- 
racy. It  is,  however,  a  dangerous  practice  to  depend 
upon  such  a  balancing  of  errors, 

It  must  further  be  borne  in  mind  that  the  common 
impurities  of  analytical  reagents  are  the  normal  constit- 
uents of  clays,  and  may  frequently  throw  out  an  accu- 
rately manipulated  analysis  several  per  cent. 

Hydrochloric  acid  and  ammonia  frequently  contain 
very  appreciable  traces  of  alumina  and  soluble  silica. 
Precipitated  calcium  carbonate  (if  ^awrence  Smith's 
alkali  determination  be  used)  cannot  be  bought  free 
from  alkali,  and  dry  sodium  carbonate  contains,  almost 
invariably,  traces  of  silica,  and  has  not  infrequently  been 
found  by  the  writer  to  contain  fine  splinters  of  parian, 
from  the  " porcelain"  linings,  possibly,  of  the  mills  in 
which  it  was  ground,  whereby  silica  and  alumina  would 
both  be  introduced  into  the  analysis. 


6  THE   CHEMISTRY   OP   POTTERY. 

Distilled  water  is  also  a  very  fruitful  source  of  diffi- 
culty ;  for  in  washing  the  bulky  precipitates  occurring  in 
clay  analyses,  traces  of  impurity  rapidly  heap  up  to 
important  factors.  Trouble  has  been  found  with  water 
which,  it  was  supposed,  had  been  distilled  with  great  care 
and  condensed  in  a  block-tin  worm  that  could  not  be  sus- 
pected. Still  the  latter  proved  to  be  the  source  of  the 
difficulty.  The  upper  coil,  into  which  the  hot  steam 
enters,  had,  in  the  course  of  two  years,  become  highly 
crystalline,  and  was  riddled  with  fine  cracks.  Into  the 
latter,  the  cooling  water  had  seeped  and  vitiated  the  dis- 
tillate. 

Such  a  defect  in  the  condensing  worm  may  readily  be 
detected  by  drawing  the  cooling  water  from  the  tub,  and 
now  blowing  the  steam  from  the  still  into  the  coil.  Pres- 
ently the  mud  and  scale  deposited  on  the  latter  are  dried 
through  the  heating  of  the  worm.  If  now  little  spots  of 
the  same  remain  persistently  damp,  it  becomes  apparent 
that  underneath  are  cracks  in  the  worm  sufficient  to  let 
the  steam  through,  and  sufficient,  also,  when  the  tub  is 
filled,  to  give  the  cooling  water  access  to  the  interior  of 
the  worm. 

In  the  case  of  clays  containing  a  larger  amount  of 
alkali,  if  potash  and  soda  be  not  separated,  but  the  titer 
of  the  chlorine  found  in  the  weighed  chlorids,  and  the 
equivalent  oxids  calculated,  it  is  important  to  give  the 
combining  weight  of  the  alkali  obtained,  so  that  the 
chemical  formula  of  the  clay  may  be  calculated,  in  case 
it  should  be  desired  to  use  it  as  or  in  a  glaze. 

Beside  the  ultimate  analysis,  a  proximate  one  is  very 


ANALYSIS   OF    MATERIALS   AND    PRODUCTS.  7 

important.  Considering  the  formation  of  clays,  it  will  be 
appreciated  that  its  various  ultimate  constituents  are 
grouped  in  a  variety  of  mixed  minerals  of  widely  differ- 
ing physical  properties,  and  that  these,  rather  than  the 
former,  determine  the  character  of  the  clay. 

An  accurate  separation  of  the  clay  into  its  various  com- 
ponent minerals  is,  in  the  present  state  of  analytical 
knowledge,  out  of  the  question;  yet,  the  well-known 
separation  of  mineral  and  quartz  sand,  by  digesting  the 
clay  in  strong  sulfuric  acid,  and  after  washing  out  the 
excess  of  acid  and  the  sulf ates  formed  by  the  decomposi- 
tion of  the  clay  substance  proper  and  the  micaceous  min- 
erals, and  removal  of  the  separated  soluble  silica  with  a 
solution  of  alkali  carbonate,  described  in  text-books 
on  analytical  chemistry,  gives  data  of  great  practical 
value. 

When  the  sand  thus  found  in  a  clay  is  not  pure  quartz, 
but,  as  is  generally  the  case,  consists  of  the  detrital  mat- 
ter of  a  great  variety  of  minerals,  it  becomes  very  diffi- 
cult to  estimate  its  probable  effect  in  its  influence  on  the 
refractory  qualities  of  a  clay,  and  the  resulting  coefficient 
of  expansion  of  the  burned  body.  For  minerals,  more  or 
less  fusible,  greatly  reduce  the  refractoriness  of  a  clay, 
while  quartz,  at  moderate  temperatures  (though  not  at 
high  ones) ,  increases  it ;  and  with  reference  to  the  coeffi- 
cient of  expansion,  the  effect  of  these  substances  is  also 
diametrically  opposite. 

As  now,  there  is  no  way  of  determining  in  a  mixture 
of  quartz  and  mineral  sand  how  much  of  the  silica  belongs 
to  the  minerals  and  how  much  to  the  quartz  itself, 


8  THE   CHEMISTRY   OF   POTTERY. 

Seger  and  Aron1  have  suggested  that,  inasmuch  as  the 
mineral  detritus  of  the  sand  is  more  or  less  fusible,  and 
plays  the  same  role  in  a  pottery  body  as  powdered  feld- 
spar, a  substance  with  the  action  of  which,  in  a  clay ,*  the 
potter  is  perfectly  familiar;  and  as,  further,  such  min- 
eral sand  is  often  mainly  feldspathic,  it  is,  for  all  practi- 
cal purposes,  sufficient  to  consider  it  feldspar. 

Hence,  the  alumina  obtained  on  analyzing  the  sand  is 
used  as  the  factor  with  which  the  silica  to  be  assigned  to 
the  mineral  sand  is  determined,  its  weight  being  multi- 
plied by  the  factor  expressing  the  proportion  of  silica  to 
alumina  in  feldspar,  and  this  amount  of  silica  being 
deducted  from  the  total  silica  of  the  sand,  the  remaining 
silica  is  called  quartz,  while  that  deducted,  together  with 
the  alumina  and  the  alkalis  and  alkaline  earths  found  in 
the  sand,  are  added,  and  designated  ^feldspathic"  sand 
or  detritus. 

The  difference  between  the  weights  of  the  constituents 
found  in  the  complete  analysis  of  the  clay  and  those  of 
the  sand,  left  by  the  sulfuric  acid  and  alkali  carbonate 
treatment,  are  calculated  as  constituting  the  "  clay  sub- 
stance." In  order  that  the  proportion  of  the  latter  may 
be  more  satisfactorily  surveyed  and  compared  with  those 
of  kaolinite,  it  is  necessary  to  reduce  them  to  a  basis  of 
100. 

The  ' '  rational  analysis, ' '  then,  looks  upon  every  clay  as 
consisting  of  a  "clay  substance"  deviating  more  or  less 
from  kaolinite,  in  which  the  alumina  is  substituted, 


iNotizblatt  des  T6pfer-u.  Ziegler-vereins,  1874,  S.  226,  and  Zwick,  Jahres- 
bericht,  1878,  S.  21,  and  1879,  S.  41. 


ANALYSIS   OF   MATERIALS   AND    PRODUCTS.  9 

to  a  greater  or  less  degree,  by  ferric  oxid,  etc.,  and 
the  combined,  water  by  alkalis  and  alkaline  earths ; 
quartz,  in  a  finely  divided  state,  and  mineral  sand,  prac- 
tically feldspathic. 

Scientifically,  it  must  be  conceded  Dr.  C.  Bischof  that 
the  result  is  ' '  ein  kiinstlich  theoretisches  Bild, ' ' !  both  on 
account  of  the  assumption  underlying  the  calculation  of 
the  mineral  sand  as  feldspar,  and  because  the  method  of 
separating  the  mineral  and  quartz  sand  from  the"  clay 
substance  is  by  no  means  an  accurate  analytical  process. 

Thus  a  pure  feldspar  (orthoclase)  treated  in  the  same 
manner  as  the  clays  are  treated,2  left  in  one  instance  a 
residue  of  81.44  percent.,  and  in  another  83.95  per  cent. 

In  the  hope  that  clays  might  be  decomposed  with  dilu- 
ted sulfuric  acid  under  pressure  under  conditions  that 
would  cause  less  action  on  feldspathic  minerals,  it  was 
found  that  two  grams  of  kaolin  treated  with  twenty  cubic 
centimeters  of  five  times  normal  sulfuric  acid  in  a  sealed 
tube  of  hard  glass,  was  only  completely  decomposed  in 
two  hours,  at  a  temperature  of  200°  C.,  conditions  that 
were  insufficient  for  decomposing  some  plastic  clays.  Yet 
a  feldspar  subjected  to  the  same  treatment,  left  after  sep- 
aration of  the  acid  and  washing  out  the  soluble  silica 
with  solution  of  sodium  carbonate,  a  residue  of  only 
82.67  Per  cent.,  practically  the  same  as  the  regular  treat- 
ment. 

Vfc 

It  is  hardly  possible  then,  with  existing  methods,  to 


1  Bockmann :  Untersuchungs  methoden,  S.  354,  1884. 

2  Fresenius :  Anleitung  zur  Quantitative!!  Chemischen  Analyse,  Sechste 
Auflage,  ii,  352,  f. 


10  THE   CHEMISTRY   OF   POTTERY. 

dissolve  the  "clay  substance"  out  of  a  clay  without  a 
loss  of  feldspar  that  may  reach  twenty  per  cent,  of  the 
amount  present. 

The  treatment  with  sulfuric  acid  even  affects  the  quartz 
to  some  extent  and  leads  to  its  removal  by  the  subse- 
quent sodium  carbonate  treatment. 

Thus  a  pure  finely  powdered  quartz  was  found  to  con- 
tain 3.75  per  cent*  silica  soluble  in  sodium  carbonate  solu- 
tion. Subjected  after  removal  of  the  same  to  the  treat- 
ment used  for  solution  of  "clay  substance,"  it  lost  3.88 
per  cent,  apparently  rendered  soluble  by  the  action  of  the 
sulfuric  acid. 

Practically,  this  division  of  the  analytical  data  of  clays 
is  of  the  greatest  service  to  the  potter,  who  is  familiar 
with  "clay,  flint,  and  spar,"  and  knows  how  to  vary 
their  proportions  for  his  various  ends.  It  has  resolved 
the  composition  of  natural  clays  into  components  with 
which  he  knows  how  to  deal,  and  made  once  unintelli- 
gible analyses  serviceable  to  him. 

The  following  may  serve  as  an  example  of  the  analy- 
sis of  a  clay,  conducted  on  these  lines.  It  is  a  kaolin 
from  Nelson  County,  Virginia. 

The  entire  clay.  The  contained  sand. 

Silica    50.02  12.62 

Alumina 35-I8  i-72 

Ferric  oxid 0.36  0.09 

Lime 0.12  0.06 

Magnesia 0.07  *         0.02 

Alkalis 3-391  i.o82 

Combined  water 10.57  °'°° 

99-71  15.59 


1  Combining  weight  43.         2  Combining  weight  34.2. 


ANALYSIS   OF   MATERIALS   AND   PRODUCTS.  II 

Rational  Analysis. 

Clay  substance ^.  84.12 

Feldspathic  detritus *.     9.04 

Quartz 6.55 

99.71 

PERCENTAGE  COMPOSITION  OF 

Clay  substance.  Feldspathic  detritus. 

Silica 44.47  67.15 

Alumina 39-79  i9-°3 

Ferric  oxid 0.32  i.oo 

Ivime 0.07  0.66 

Magnesia    0.02  0.22 

Alkalis 2.75  H-95 

Combined  water 12.58  o.oo 


100.00  100.00 

Should  the  clay  contain  limestone  detritus  and  appre- 
ciable amounts  of  uncombined  ferric  oxid  or  alumina, 
it  is  proper  to  extract  these  with  dilute  hydrochloric  acid, 
and  place  them  in  the  rational  analysis  by  themselves. 
Similarly,  if  the  clay  contain  soluble  silica,  this  should 
be  extracted,  and  added  to  the  quartz. 

When  this  is  done,  as  before  pointed  out,  the  per- 
centage formula  of  the  "clay  substance,"  as  a  rule, 
closely  approximates  kaolinite^the  variation  found  from 
the  latter  being  in  the  partial  substitution  of  alumina  by 
ferric  oxid  and  of  the  combined  water  by  alkalis  and 
alkaline  earths.  These  variations,  however,  the  potter 
can  easily  learn  to  allow  for,  by  the  now  simple  compar- 
ison with  the  percentage  composition  of  kaolinite. 

It  will  occur  to  the  chemist,  that  as  the  detritus  of 
micas  must  be  a  constant  constituent  of  clays,  and  as  the 
dust  of  these  minerals  is  practically  all  decomposable  by 
sulfuric  acid,  and  as  their  presence  in  the  "clay  sub- 


12 


THE   CHEMISTRY   OF   POTTKRY. 


stance"  will  vary  its  percentage  composition  just  as 
described  above,  it  would  seem  both  justifiable  and  ser- 
viceable to  assume  that  the  alkalis  and  alkaline  earths  of 
the  ' '  clay  substance' '  are  contained  in  such  combination, 
and  using  the  average  percentage  of  alkali  in  muscovite 
as  a  measure,  deduct  from  each  constituent  a  corres- 
ponding amount  as  going  to  make  up  so  much  mica. 

The   "rational  analysis"  of  the   above  clay   would, 
under  this  consideration,  be 

Clay  substance 60.23  Per  cent. 

Mica    23.89       " 

Feldspar    9.04       ' ' 

Quartz 6.55       " 


Feldspar. 

67.15 

19.03 

1. 00 

0.66 

0.22 

H-95 


99.71 
the  percentage  composition  being 

Clay  substance.        Mica.  Feldspar.          Quartz. 

Silica 43-75  46.30  67.15  loo.oo 

Alumina 39.64  4o-*7 

Ferric  oxid 1.13 

Lime 0.25 

Magnesia 0.08 

Alkalis    9.67 

Combined  water 16.61  2.40 

100.00      100.00      100.00       100.00 

This  extension  of  the  "rational  analysis"  introduces  a 
second  hypothetical  factor,  which  may  often  be  justified 
in  a  primary  clay  where  mineralogical  examination  con- 
firms such  a  division,  but  beyond  this,  even  the  practical 
needs  of  the  potter  do  not  demand  it,  as  the  micas  are 
not  used  as  fluxes  in  ceramic  industries,  and  their  hypo- 
thetical presence  would  convey  less  meaning  to  him  than 
the  percentage  composition  of  a  "  clay  substance"  easily 
comparable  with  kaolinite. 


ANALYSIS   OF   MATERIALS   AND    PRODUCTS.  13 

Clay  analyses  should  always  be  calculated  to  the  basis 
of  the  sample,  dried  at  120°  C.,  as  analyses  showing  vary- 
ing amounts  of  moisture  are  not  readily  comparable. 
Upon  the  analyses  of  the  other  minerals,  oxids,  and  salts 
used  by  the  potter,  it  is  not  necessary  to  dwell.  Especial 
impurities,  for  which  it  is  alone  necessary,  often,  to  make 
examination,  will  be  mentioned  when  the  substances 
*  themselves  are  discussed. 

Of  ceramic  products,  the  analysis  of  the  pottery  bodies 
is  conducted  similarly  to  that  of  a  clay,  care  being  taken 
in  the  preparation  of  the  sample  that  every  trace  of  glaze, 
etc.,  be  chiseled  or  ground  off.  The  calculation  of  the 
composition  of  a  mass  or  paste  for  producing  the  same 
from  known  clays  and  minerals  is  readily  done  on  well- 
known  stoichiometric  rules.  A  practical  burning  trial 
will  then  show  how  the  mass  obtained  from  the  analysis 
will  have  to  be  modified  to  allow  for  the  individual  phys- 
ical characteristics  of  the  clays  used. 

The  obtaining  of  samples  of  enamels  and  glazes  from 
finished  pieces  of  ware  is  often  attended  with  considerable 
difficulty  ;  inasmuch  as  these  are  frequently  very  thin,  or 
the  body  of  the  ware  is  so  soft  that  it  chips  off  with  them, 
contaminating  the  sample. 

The  most  practical  way  of  obtaining  them  is  to  bed  the 
piece  from  which  the  sample  is  to  be  obtained  in  clay,  on  a 
convenient  table,  surrounding  the  piece  with  large  sheets 
of  glazed  paper,  to  catch  the  splinters  of  the  glass  as  they 
are  struck  off.  The  enamel  or  glaze  is  now  dressed  off 
with  small  chisels  of  hardened  steel,  driven  with  a  light 
hammer.  As  the  chisels  soon  dull,  a  sufficient  number 


14  THE   CHEMISTRY  OF   POTTERY. 

of  them  must  be  kept  on  hand  to  carry  on  the  work.  The 
blows  of  the  hammer  must  be  so  regulated  that  the  chisel 
does  not  cut  into  the  body  of  the  ware.  When  sufficient 
of  the  sample  has  been  dressed  off,  it  is  carefully  swept 
together  with  a  camel's  hair  pencil,  and  probed  with  a 
bright,  clean  magnet  until  all  the  iron  introduced  by  the 
chisels  is  extracted.  It  is  then  ground  fine  in  the  agate 
mortar  for  analysis. 

From  the  usual  components  of  potters'  glazes,  given  in 
a  later  chapter,  the  chemist  will  understand  what  class 
of  substances  it  is  necessary  to  look  for,  and  what  pre- 
cautions must  be  taken  in  the  analyses.  The  almost  con- 
stant presence  of  reducible  metals  would  seem  to  make 
these  analyses  difficult,  on  account  of  the  unavoidable 
fusions  in  platinum  ;  but  by  unlocking  the  glasses  with 
a  liberal  amount  of  alkali  carbonate,  and  keeping  the 
crucible  well  up  in  the  flame,  no  reduction  need  be 
feared. 

The  frequent  presence  of  considerable  amounts  of  bo- 
racic  acid  in  these  silicates  formerly  presented  an  almost 
unsurmountable  difficulty  to  accurate  analysis,  but 
Gooch's  method  of  determining  this  substance1  has  over- 
come this  trouble ;  yet  we  have  still  to  deal  with  the  most 
difficult  phase  of  the  determination,  for  the  unlocking  of 
boro-silicates  containing  frequently  a  very  high  per- 
centage of  reducible  metal,  compels  fusion  with  large 
amounts  of  alkali,  and  the  following  acidification  fills  the 
retort  with  a  great  mass  of  troublesome  salts. 


1  Bulletin  of  the  U.  S.  Geological  Survey,  No.  43,  p.  64.    See  also  J.  Anal. 
Appl.  Chem.,  2,  86. 


CHAPTER  II. 
PHYSICAL  AND  EMPIRICAL  TESTS. 


HE  physical  properties  of  clays  play  so  im- 
portant a  part  in  their  use  and  in  deter- 
mining the  character  of  the  products  which 
can  be  made  from  them,  that  the  chemical 
analysis,  by  itself,  is  altogether  insufficient  to  tell  what 
qualities  may  be  expected  from  such  materials.  Kven 
in  the  matter  of  the  color  to  be  obtained  on  burning  the 
clay,  which  would  seem  to  hang  most  closely  together 
with  its  chemical  composition,  the  analysis  leaves  us, 
except  in  extreme  cases  and  in  a  general  way,  in  doubt. 
The  coloring  of  clays  in  the  fire  is  due,  primarily,  to 
the  presence  of  iron  oxid,  the  tint  being  modified  by 
the  amount  of  lime  occurring  and  influenced,  occasionally 
by  the  presence  of  manganese,  as  also  by  the  chemical 
quality  of  the  flame  during  burning.  But  the  depth  of 
color  of  a  burned  clay  is  not  at  all.  in  proportion  to  the 
contained  iron  oxid ;  a  secondary  clay  may  contain 
considerably  less  iron  than  a  kaolin  and  yet  burn  quite 
yellow,  while  the  latter  burns  snowy  white.  All  depends 
upon  the  combinations  in  which  the  iron  is  held,  and  as 
long  as  we  are  unable  to  accurately  separate  the  minerals 
of  a  clay,  and  subject  them  individually  to  examination, 
the  chemical  determination  of  the  gross  amounts  of  the 


l6  THE   CHEMISTRY   OF    POTTERY. 

coloring  oxids  will  give  us  but  a  very  rough  idea  of  the 
tints  we  may  expect. 

To  formulate  a  systematic  set  of  physical  and  empir- 
ical tests  to  characterize  a  clay  is,  on  account  of  the  widely 
differing  needs  of  clay-workers,  a  very  difficult  thing. 
Yet  obvious  operations  connected  with  its  general  use 
must  be  undertaken,  and  the  conditions  of  the  trials  and 
their  resulte  must  be  described,  as  far  as  possible,  in 
terms  of  physical  and  chemical  measurement  at  com- 
mon command.  If  this  be  done,  inferences  can  be  drawn 
from  the  experiments  sufficiently  close  to  the  behavior  of 
the  clay  under  working  conditions,  to  reliably  forecast  its 
serviceability  or  worthlessness  for  this  or  that  purpose. 

In  this  view,  at  least  the  following  tests  should  be 
made: — 

1.  The  fineness  of  grain  of  the  constituents  of  the  clay, 
measured  by  passing  it  dry  or  suspended  in  water  through 
sieves  of  different  mesh,  or  washing  it  with  water  at  dif- 
ferent measurable  speeds  of  the  wash-water,  recording  the 
proportion  of  each  grade  of  fineness  measured. 

2.  The   plasticity,    expressed   numerically,    by   some 
measurable  quality  connected  with  it. 

3.  The  binding  property,  given  as  tensile  strength. 

4.  A  firing  under  definite  conditions  of   the  chemical 
character  of  the  flame,  with  record  of  the  duration  of  the 
fire  and  the  temperature  reached,  measured  in  a  satisfac- 
tory pyrometric  standard,  to  attain  a  specific  hardness. 

5.  The  porosity  of  the  product  burned  as  given,  deter- 
mined by  its  water-absorption. 

6.  The  shrinkage  from  the  clay  state  in  which  the 
material  is  formed  to  its  burned  condition. 


PHYSICAL   AND    EMPIRICAL  TESTS.  17 

7.  Its  coefficient  of  expansion,  when  burned  as 
described,  measured  empirically  by  melting  on  it,  at  a 
heat  lower  than  that  of  its  original  baking,  a  glaze  of 
definite  chemical  composition,  noting  if  in  time,  the  glaze 
alone  crack,  a  phenomenon  called  "crazing,"  or  if  it 
shatter  the  body  or  fly  off  at  the  edges,  tearing  the  clay 
along  with  it,  a  phenomenon  opposite  to  t;hat  of ' '  crazing' ' 
and  called  "shivering,"  which  result  from  either  a  too 
great  or  too  small  coefficient  of  expansion  of  the  clay  as 
compared  with  that  of  the  glaze. 

It  is  of  course,  quite  important,  that  the  sample  for 
these  physical  and  empirical  tests  be  properly  averaged, 
and  in  fact  the  same  from  which  the  portioia  for  the 
chemical  analysis  is  taken. 

Much  was  hoped  of  the  mechanical  analysis  of  clays,1 
by  clay- workers,  in  the  direction  of  a  serviceable  separa- 
tion of  the  material  into  its  constituent  minerals  by  their 
hydraulic  values,  as  a  preparation  of  the  sample  for 
chemical  analysis  and  many  separations  were  made  with 
the  apparatus  of  Schone,2  modified  by  Schiitze.3 

Disappointment  at  the  results,  in  the  light  of  the  mis- 
applied and  over-great  expectations  has  reduced  this 
branch  of  operations  to  mere  sieve-analysis  and  prepara- 
tion of  the  sample  for  the  analytical  and  physical  tests  of 
the  clay,  as  the  needs  of  the  particular  industry,  in  which 
it  is  likely  to  be  used  would  indicate. 


1  For  an  excellent  discussion  of  the   principles  and  methods  see  Wiley 
Principles  and  Practice  of  Agricultural  Analysis,  Vol.  i,  Part  Fourth,  p.  171. 

2  Zeitschrift  fiir  analytische  Chemie,  7,  29. 
•SNotizblatt  fiir  Fabrikation  von  Ziegeln,  etc.,  1872,  88. 


1 8  THE    CHEMISTRY   OF   POTTERY. 

The  appearance,  geological  origin  or  a  preliminary 
burning  trial,  giving  the  general  use  to  which  a  clay  may 
be  put,  the  sample  is  prepared  by  passing  it  dry  or  more 
commonly  by  washing  it  through  such  a  sieve  as  the 
mechanical  preparation  of  the  clay  for  the  industry 
would  demand. 

The  material  remaining  on  the  sieve  used  should  be 
weighed  and  described.  That  passing  through  is  dried 
and  used  for  the  various  tests. 

There  is  not,  as  yet,  an  altogether  satisfactory  meas- 
ure of  the  plasticity  of  clays  ;  though  the  possibility  of 
giving  direct  numerical  expression  to  this  subtle  and 
most  valuable  property  would  be  of  great  practical  value. 

The  best  determination  that  can  thus  far  be  made  is 
based  on  the  observation  that,  in  the  main,  the  greater 
the  plasticity  of  a  clay  the  larger  the  amount  of  water 
required  to  bring  it  to  a  definite  degree  of  softness,  at 
which  it  can  be  worked. 

In  order  to  determine  this  point,  apparatus  have  been 
devised  for  pushing,  with  a  fixed  load,  a  wire,  rod  or 
thin-walled  cylinder  to  a  certain  depth  and  within  a  cer- 
tain time,  into  the  softened  clay. 

The  proportion  of  water  required  to  soften  one  hun- 
dred parts  of  the  dry  clay  to  the  requisite  emollescence 
is  taken  as  the  direct  measure  of  the  plasticity. 

The  apparatus  necessary  to  determine,  if  the  requisite 
softness  of  the  clay  is  attained,  which  is  most  conve- 
nient, inasmuch  as  it  is  already  in  use  for  determining 
the  time  of  setting  of  Portland  cement,  is  Vicat's  needle.1 


1  Transactions  of  the  American  Society  of  Civil  Engineers,  1893.    Max 
Gary  :  The  Testing  of  Portland  Cement. 


PHYSICAL   AND    EMPIRICAL   TESTS.  IQ 

It  has  a  wire  of  circular  cross-section,  the  end  cut  at 
right  angles  to  its  axis,  with  an  area  of  one  square  mil- 
limeter. The  wire  is  over  four  centimeters  long  and 
attached  to  the  end  of  a  rod,  suitably  guided,  which 
weighs  300  grams. 

The  softened  clay  is  well  ' '  wedged' '  to  make  it  per- 
fectly homogeneous,  and  pressed  into  a  ring  four  centi- 
meters deep,  set  on  a  glass  plate  under  the  needle  and 
struck  off  level  with  a  steel  spatula.  The  needle  is  then 


VICAT'S  NEEDLE. 

allowed  to  penetrate  the  clay,  and  if  within  five  minutes 
it  sinks  to  a  depth  of  four  centimeters  into  the  same,  the 
clay  is  of  the  proper  consistence ;  if  not,  it  is  either  made 
stiff er  or  softened,  as  the  case  may  be. 


20  THE   CHEMISTRY   OF   POTTERY. 

A  weighed  sample  of  the  mass  of  proper  consistence  is 
dried  and  the  proportion  of  water  to  100  parts  of  dry 
clay  is  calculated,  the  figure  being  used  as  the  direct 
index  of  the  plasticity. 

The  binding  power  of  clays  is  determined  by  tearing 
well-dried  specimens  formed  in  cement-briquette  molds, 
the  results  being  expressed  in  the  weight  in  grams  per 
square  centimeter  sufficient  to  break  the  clay. 

Any  of  the  standard  machines  for  testing  the  tensile 
strength  of  cement1  will  answer  the  purpose. 

It  is  necessary,  however,  to  form  the  test  briquettes 
very  carefully,  as  they  are  liable  to  contain  flaws,  par- 
ticularly with  very  plastic  clays,  which  make  it  difficult 
to  get  concordant  data. 

The  clay  must  be  well  "wedged"  and  beaten  out  into 
a  slab  rather  thicker  than  the  mold.  By  means  of  a 
bent  tin  stamp  a  piece  of  such  dimensions,  that  it  will 
very  nearly  fit  the  mold,  is  cut  out,  into  which,  after 
well  oiling  the  latter  and  setting  it  on  a  dry  plaster  of 
Paris  slab,  it  is  beaten,  and  the  excess  of  clay  cut  off 
with  a  thin  wire.  The  mold  is  then  slipped  off  the  bri- 
quette, care  being  taken  not  to  disturb  its  shape. 

After  the  air-dried  pieces  have  been  broken  in  the 
machine,  the  surfaces  of  fracture  must  be  measured 
before  calculating  the  breaking  load,  that  due  allowance 
be  given  for  the  shrinkage  of  the  clay  in  drying. 

The  binding  power  of  clays  is  not  necessarily  propor- 
tional to  their  plasticity,  as  one  would  naturally  sup- 


i  Journal  of  the  American  Chemical  Society,  16,  161, 1894.    Thos.  B.  Still- 
man  :  The  Chemical  and  Physical  Examination  of  Portland  Cement. 


PHYSICAL   AND    KMPIRICAI,   TESTS.  21 

pose.  The  New  Jersey  ball  clay  describe4  in  a  subse- 
quent chapter  is  in  striking  illustration  of  this  fact. 

Clays  which  require  a  large  amount  of  water  to  render 
them  workable,  and  therefore  show  considerable  shrink- 
age in  the  clay  state,  on  drying,  but  which  when  dry  are 
of  low  tensile  strength,  are  liable  to  be  very  troublesome, 
particularly  in  the  making  of  heavy  and  of  dust-pressed 
wares  by  ''checking,"  that  is,  showing  surface  cracks 
or  "dunting"  (cracking  through).  For  drying,  taking 
place  from  the  surface  of  the  piece,  the  material  must  be 
able  to  stand  the  strain  of  its  shrinkage  on  a  more  slowly 
contracting  center. 

Where  clays  are  deficient  in  this  particular,  great  pre- 
cautions have  to  be  taken  to  prevent  the  loss  of  ware 
made  with  them  in  rapid  drying,  which  often  involves 
so  much  cost  and  care  that  the  use  of  the  material 
becomes  out  of  the  question. 

Although  half  a  dozen  disks  of  the  properly  sampled 
clay,  of  the  diameter  and  twice  the  thickness  of  a  silver 
quarter,  would  answer  to  determine  all  that  it  is  desired 
to  know  of  the  firing  of  a  clay,  this  test  cannot  be 
made  over  a  Bunsen  or  blast-flame,  with  the  pieces 
packed  in  a  platinum  crucible,  even  with  the  help  of  the 
Brdman  furnace ;  the  difficulty  being  that  with  the  sim- 
ple appliances  of  the  ordinary  laboratory,  it  is  not  possi- 
ble to  get  a  sufficiently  large  zone  in  which  the  chemical 
character  of  the  flame  remains  the  same  through  a  suffi- 
cient length  of  time,  and  in  which  the  temperature  is 
likewise  the  same  and  can  be  observed  or  measured  with- 
out altering  the  conditions. 


22  THE   CHEMISTRY   OF    POTTERY. 

Hence  it  becomes  necessary  to  make  use  of  larger 
pieces  of  apparatus,  which  in  the  greater  amount  of  time 
required  in  their  heating,  give  a  better  opportunity,  in 
the  admission  of  fuel  and  regulation  of  the  draft,  to  get 
the  quality  of  flame  desired,  and  further,  by  their  slower 
and  more  regular  advance  in  temperature,  give  the  clay 
pieces,  which  are,  in  themselves,  very  poor  conductors 
of  heat,  the  opportunity  of  progressing  in  their  entirety 
through  the  changes  caused  by  advancing  temperature, 
without  a  shrinkage  of  the  periphery  on  the  centers, 
causing  distortion  and  unequal  tension. 

The  ordinary  assayer's  muffle  will  answer  the  purpose 
of  firing  very  well,  whether  it  be  heated  with  coke,  gas, 
or  gasoline ;  but  the  draught  used  should  be  the  natural 
one  of  a  good  chimney,  regulable  by  a  damper  and  not 
the  forced  draught  of  a  fan  or  bellows,  as  the  quality 
of  the  flame  is,  in  the  latter  case,  too  uncertain  and  diffi- 
cult to  regulate. 

Where  gas  is  available,  the  most  convenient  furnace 
is  one  made  after  designs  of  Seger,  by  Geith,  in  Coburg, 
Germany,  consisting  of  a  ring  of  eight  Bunsen  burners, 
the  flames  playing  into  a  fire-clay  furnace  and  forced  by 
the  connection  with  the  chimney  in  the  bottom,  to  play 
over  a  ring  and  down  on  the  crucible  or  ' '  saggar' '  before 
passing  into  the  flue.  In  this  way,  by  the  up  and  down 
passage  of  the  flame,  a  large  zone  of  uniform  tempera- 
ture is  well  attained.  The  arrangement  will  be  well 
understood  from  the  sketch  of  a  similar  contrivance 
which  can  easily  be  extemporized  with  good  fire-brick 
and  tile  by  a  practical  brick -layer. 


PHYSICAL   AND    EMPIRICAL  TESTS. 


For  attaining  high  heats,  it  is,  of  course,  imperative 
that  the  gas  pressure 
be  sufficient,  a  con- 
dition often  only  to' 
be  had  in  the  even- 
ing, as  most  of  our 
gas  companies  are 
conducted. 

The  temperature 
at  which  different 
clays  should  be  fired 
and  the  melting  of 
glazes  on  the  fired 
pieces,  to  empirical- 
ly test  their  coeffi- 
cients of  expansion, 
can  best  be  explained^ 
in  connection  with 
the  considerations 
applying  to  their  use , 
and  is  deferred  to  the 
respective  chapters 
on  the  different  wares 
as  the  results  of  ex- 
perimental burning  are  only  of  value  when  looking  to  the 
conditions  of  definite  products,  and  when  the  heats  to 
which  the  trials  have  been  subjected  are  measurable  and 
can  be  reproduced  and  controlled,  to  a  similar  degree,  in 
the  kilns  of  the  potter.  The  subject  of  pyrometry  be- 
comes one  of  the  greatest  importance  in  this  connection ; 
to  it,  the  subsequent  chapter  will  be  devoted. 


Scale,  fc  to  1 


GAS  TRIAL  KILN. 


24  THE   CHEMISTRY   OF   POTTERY. 

In  the  case  of  clays  intended  for  making  floor-tile  and 
paving-brick,  and  for  ware  which  is  to  be  partly  glazed 
and  exposed  to  the  influences  of  the  weather,  it  is  im- 
portant to  know  how  porous  it  still  is  after  receiving  the 
proper  fire.  It  is  sufficient,  for  this  purpose,  to  deter- 
mine the  weight  of  water  that  the  burned  specimen  will 
absorb. 

In  making  the  test  it  is  important  not  to  immerse  the  en- 
tire piece  in  water,  as  this  would  seal  the  pores  and  make 
it  very  difficult  to  get  rid  of  the  air  in  the  interior.  One 
face  of  the  specimen  must  be  left  dry,  and  the  water  al- 
lowed to  rise  by  capillarity,  the  piece  not  being  lowered 
under  the  surface  of  the  water  until  all  its  air  has  been 
expelled.  It  is  superficially  wiped  off  before  weighing. 

As  all  clays  shrink  in  the  fire,  but  show  the  greatest 
variation  in  this  property,  and  as  articles  made  from  them 
are  generally  required  to  come  to  definite  sizes,  hollow- 
ware  being  made  to  certain  capacities,  sanitary- ware 
having  to  fit  metallic  fittings,  and  brick  and  tile  being 
required  of  standard  sizes,  a  description  of  the  physical 
properties  of  a  clay  involves  a  statement  of  the  amount 
of  its  shrinkage  from  the  plastic  condition  in  which  it  is 
worked  to  the  condition  reached  at  the  temperature  in 
which  it  is  burned ;  so  that  from  it  the  artisan  will  be 
enabled  to  calculate  the  sizes  of  his  molds  or  dies. 

If,  for  the  burning  trials,  disks  of  about  one  and  one- 
half  inches  in  diameter  and  three-sixteenths  of  an  inch 
thick,  be  beaten  out  from  the  plastic  clay  on  clean  bats 
of  plaster  of  Paris,  the  surface  of  the  disks  being  smoothed 
with  a  knife  or  steel  spatula  and  two  dots  impressed  or 


PHYSICAL   AND    EMPIRICAL   TESTS.  25 

lines  drawn  into  the  soft  clay,  just  twenty-five  millimeters 
apart,  the  shrinkage  of  the  clay  is  found  with  sufficient 
accuracy  after  the  trials  have  been  burned,  as  in  the  sec- 
ond measurement,  quarter  millimeters  are  easily  estima- 
ted so  that  the  result  may  be  given  in  per  cents. 

Articles  of  which  the  greatest  precision  in  size  is  re- 
quired, flooring  and  wall-tile  and  fine  pressed  brick,  are 
not  formed  from  clay  in  the  plastic  state,  but  from  fine 
clay-dust  containing  from  eight  to  twelve  per  cent,  mois- 
ture, so  that  it  just  packs  when  squeezed  between  the 
fingers. 

The  shrinkage  of  the  dust-pressed  and  plastic-pressed 
clay  will  vary  somewhat,  and  it  is  useful  to  make  for  the 
burning  a  few  dust-pressed  disks,  beside  the  plastic 
ones.  This  is  easily  done  in  a  large  diamond  mortar, 
twenty-five  millimeters  in  diameter.  It  must  be  well 
cleaned  and  oiled,  the  ring  filled  one-third  to  one-half 
full  of  the  slightly  dampened  clay-dust,  the  pestle  insert- 
ed and  pushed  down,  then  struck  twice  gently  with  the 
hammer  in  order  to  pack  the  clay,  but  still  let  out  the  air, 
and  finally  struck  two  sharp  blows  with  the  hammer  to 
compress  and  seal  the  disk.  It  is  then  carefully  pushed 
out,  and  after  being  dried  and  fired  to  the  established 
heat,  is  measured  to  quarter  millimeters  with  a  pair  of 
calipers  and  the  shrinkage  recorded  in  per  cents. 


CHAPTER  III. 
PYROMETRY. 


HE  subject  of  pyrometry  is  one  of  the 
most  vital  interest  to  an  industry  making 
use  of  the  greatest  range  of  high  tem- 
peratures, and  risking  large  quantities 
of  ware  to  an  operation  which  will  make  or  mar  it,  unless 
the  heat  desired  be  attained  within  a  very  narrow  range 
of  variation. 

Potters  use,  for  their  guidance  in  determining  if  the 
requisite  high  temperatures  have  been  reached,  empiric 
trial-pieces  made  of  materials  used  in  their  manufacture, 
the  behavior  of  which  in  the  fire  they  have  become 
familiar  with  in  the  course  of  their  practical  experience. 
Such  trial-pieces  are  usually  rings  of  clay  or  shards  of 
baked  clay  coated  with  glaze  mixtures,  fusible  clay,  or 
feldspar. 

Usually  these  empiric  standards  serve  their  purpose 
very  wrell  in  the  hands  of  experienced  men,  though  they 
fail  at  times,  even  under  such  use,  through  mechanical 
and  chemical  changes  of  the  materials,  which  nothing 
but  careful  analyses  would  detect  and  which  are  usually 
not  made. 

It  will  be  appreciated  that  the  personal  factor  is  a  very 
great  one  in  the  judging  of  such  empiric  pyrometric  trials, 


PYROMKTRY.  27 

and  it  is  this  that  the  burner  of  better  classes  of  ware,  par- 
ticularly, turns  to  his  account,  in  making  himself  indis- 
pensable to  a  manufacturer,  who  is  loth  to  entrust  the 
dangerous  operation  of  burning  to  one  who  has  first  to 
make  his  experience  with  the  trials. 

But  even  in  the  hands  of  the  experienced  burner,  they 
have  the  disadvantage  that  they  do  not  enable  him,  ex- 
cept in  a  very  imperfect  way,  to  strike  other  tempera- 
tures with  which  he  has  not  previously  worked.  He  is 
therefore  helpless  under  all  but  the  limited  conditions  of 
his  immediate  experience. 

As  the  trials  of  different  establishments  bear  no  rela- 
tion to  each  other,  thejr  experiences  in  firing  are  not 
comparable,  and  they  cannot  be  mutually  helpful,  even 
when  there  is  an  entirely  cordial  disposition  to  be  so. 

As  the  first  principles  of  systematic  chemical  investi- 
gation involve  the  use  of  standards  that  are  comparable 
and  easily  attained,  it  would  be  useless  to  go  into  the 
present  practical  kiln  trials  of  our  potters  beyond  men- 
tioning them  in  connection  with  the  different  classes  of 
manufactures,  which  will  be  done  in  the  appropriate 
places. 

It  is  necessary,  however,  to  carefully  consider  the  con- 
ditions imposed  upon  pyrometric  observations  in  pottery 
kilns,  in  order  to  select  the  best  means,  of  those  pro- 
posed, for  making  them. 

The  first  and  most  obvious  condition  is  that  the  pyrom- 
eter should  be  of  such  positive  action  that  there  enter 
no  personal  lactor  into  its  reading.  That  while  it  be 
sufficiently  wide  in  range  to  cover  all  temperatures  of 


28  THE   CHKMISTRY   OF   POTTKRY. 

ceramic  operation,  the  divisions  must  be  so  close  that  the 
progress  of  physical  change  of  clays  and  glazes  made  in 
the  fire  be  not  beyond  the  observable  progress  of  the 
pyrometer. 

These  obvious  conditions  rule  out  all  optical  pyrome- 
ters which  have  thus  far  been  made. 

Furthermore,  kiln-men,  even  if  they  be  teachable, 
have  to  do  rough  work,  and  cannot  be  expected  to 
manipulate  delicate  instruments  with  hands  roughened 
and  sense  of  touch  blunted  by  coal-shoveling. 

Pottery  kilns  are  large  affairs,  having  commonly  an 
internal  diameter  of  sixteen  feet  and  a  height  of  fourteen 
feet.  The  heat,  proceeding  from  a  number  of  fires,  can 
only  be  made  to  progress  uniformly,  by  observation  at  a 
number  of  points,  which  are  seldom  less  than  four,  and 
generally  about  eight.  These  points  must  be  average 
ones,  as  far  as  temperature  conditions  are  concerned,  and 
can  hardly  be  nearer  than  from  four  to  six  feet  to  the 
outer  circumference  of  the  kiln.  Pyrometers  whose  in- 
dications niUvSt  be  transmitted  through  this  distance  of 
widely  varying  temperatures  in  order  to  be  read  on  the 
outside  are  difficult  to  place,  and  their  readings  become 
complicated  with  correcting  factors  that  are  hard  to  fix. 

If  the  expense  of  the  appliance  be  such  that  one  can- 
not be  left  in  each  place  of  observation,  but  one  or  two 
instruments  must  be  used  to  make  all  the  measurements 
of  perhaps  several  kilns;  the  length  of  time  consumed  in 
the  work  will  be  such  that  at  the  critical  time  all  neces- 
sary observations  and  regulation  of  the  fire  by  them  can- 
not be  made  with  sufficient  expedition  ;  for  upon  open- 


PYROMETRY.  29 

ing  the  kiln  for  the  introduction  of  the  instrument,  the 
invariable  suction  of  a  strong  draught  of  cold  air  to  the 
point  where  it  is  to  be  placed  results  and  so  far  cools  that 
locality  that  some  time  must  elapse,  after  all  has  again 
been  made  tight,  before  a  reading  can  be  taken. 

Another  consideration,  inherent  in  almost  all  condi- 
tions where  pyrometric  determinations  become  desirable, 
is  very  frequently  lost  sight  of ;  namely,  that  heat  is  re- 
quired to  do  work  in  a  body  which,  through  its  mass  or 
poor  conductivity,  responds  but  slowly  to  the  impact  of 
the  flashes  of  the  former.  Through  this  comparatively 
slow  response,  the  body  averages  within  itself  the  effect 
of  heat  waves  of  widely  differing  temperatures  which  it 
is  constantly  receiving. 

Now,  a  pyrometer  which  is  not  similarly  capable  of 
averaging  the  heat  impressions  received  will  be  too  sen- 
sitive ;  many  readings  will  have  to  be  made  in  order  to 
get  the  average,  or  these  will  have  to  be  recorded  as  a 
curve,  by  a  suitable  instrument.  The  former  occupies 
too  much  time  ;  the  latter  involves  the  exposure  of  a 
delicate  mechanism  to  places  and  conditions  that  are 
very  destructive  to  it. 

One  fact  has  done  much  to  retard  real  progress  in 
pyrometry ;  namely,  the  fundamental  association,  in 
most  minds,  of  the  thermometric  degrees  of  the  mercury 
thermometer  as  something  inseparable  from  all  tempera- 
ture measurements. 

The  great  utility  of  using  the  expansion  of  liquids, 
where  the  range  of  temperature  makes  it  admissible, 
accounts  for  this  very  natural  error.  But  even  in  cer- 


3O  THE   CHEMISTRY   OF   POTTERY. 

amics,  where,  perhaps,  the  greatest  range  of  heat  requir- 
ing exact  control  is  used,  not  self-regulable  as  is,  to  a 
certain  extent,  the  case  in  many  chemical  and  metallur- 
gical processes,  there  is  no  occasion  to  pass  from  the 
thermometric  scale  to  higher  heats  by  a  regularly 
graduated  system,  and  the  attempts  to  extend,  by  calcu- 
lation and  interpolation  by  over  2,000°,  a  system  which 
gives  data  for  less  than  400,  have  awakened  false  and 
unattainable  aims. 

The  connecting  of  pjTometric  with  the  common  ther- 
mometric system  can,  and,  in  any  case,  must  be  done  by 
the  calorimeter  ;  but  the  former  need  not,  therefore,  and 
should  not  be  divided  by  thermometric  degrees. 

The  difficulties  of  pyrometry  and  the  varying  condi- 
tions under  which  it  must  be  executed  may  make  instru- 
ments based  on  a  variety  of  manifestations  of  physical 
change,  varying  positively  under  increasing  heat  neces- 
sary. The  expansion  of  gases,  the  decreasing  electrical 
conductivity  of  metallic  circuits,  the  melting  down  of  pyro- 
scopes  of  progressive  variation  in  chemical  composition, 
may  all  be  successfully  employed  for  the  purpose  in  dif- 
ferent places,  but  only  the  latter  have  maintained  the 
promise  of  permanent  success  under  the  difficult  condi- 
tions of  ceramic  pyrometry,  as  enumerated. 

The  first  of  such  melting  pyroscopes  that  gave  good 
service  and  can  still  be  employed  under  proper  precau- 
tions and  in  certain  range,  are  the  metals  silver  and  gold 
and  their  alloys,  and  the  alloys  of  gold  with  platinum,  as 
suggested  by  Prinsep.1 


1  Philosophical  Transactions,  1828,  p  79. 


PYROMETRY.  31 

These  are  made  by  weighing  off  accurately  the  rela- 
tive proportions  of  the  pure  metals,  in  the  form  of  wire, 
and  then  combining  the  portions  by  thorough  fusion 
before  the  blowpipe.  The  alloy  beads  are  hammered 
to  flat  disks,  the  melting  of  which  is  easily  observed  by 
their  collapsing  and  drawing  into  the  bead-form.  The 
alloys  usually  used  are  the  following  :  their  melting- 
points  were  determined  by  Erhard  and  Schertel.1 

Percentages.  Melting-  Percentages.  Melting- 
Gold.  Silver.  point.  Gold.  Platinum.  point, 
oo  loo  954°C.  95  5  noo°C. 
20  80               975  90  10  1130 
4o  60                995  85  15  1160 
60  40  1020  80  20  1190 
80  20  1045 
100  oo  1075 

On  account  of  the  difficulty,  however,  of  obtaining 
amounts  of  alloys  of  absolutely  uniform  composition, 
these  can  only  be  used  in  small  plates  one  or  two  deci- 
grams in  weight,  which  are  not  observable  at  any  dis- 
tance, particularly  not  in  luminous  surroundings,  and 
require  their  extraction  from  the  places  of  exposure  for 
observation  Furthermore,  on  account  of  their  becom- 
ing crystalline  in  a  very  slowly  rising  heat,  and  the 
leaching  out  from  the  crystalline  mass  of  drops  virtually 
different  in  composition  and  melting-point  from  those  of 
the  entire  alloy,  it  is  not  possible  to  leave  a  number  of 
them  in  the  kiln  and  withdraw  them  one  by  one  with  the 
rise  in  heat,  for  purposes  of  observation,  but  they  must 
be  introduced  when  the  heat  is  suspected  to  be  near  the 


1  Jahrbuch  fiir  das  Berg-und  Huttenwesen  in  Sachsen, 


32  THE    CHEMISTRY   OF   POTTERY. 

point  wished,  remain  until  they  have  the  full  heat  of  the 
surroundings,  and  then  be  withdrawn  for  observation. 
This  makes  too  much  manipulation  for  practical  work, 
and  the  small  alloy  plates  are  frequently  lost,  entailing, 
on  account  of  their  cost,  considerable  expense. 

The  difficulty  of  the  changing  physical  character  of  the 
gold  platinum  alloys  is  particularly  troublesome  when 
the  latter  metal  amounts  to  twenty  per  cent,  and  over,  so 
that  considerable  uncertainty  attaches  to  the  tests  at 
higher  temperatures. 

The  advantages  of  the  Prinsep  alloys  as  bodies  prac- 
tically uninfluenced  by  oxidizing  or  reducing  atmos- 
pheres, melting  down  at  regular  sufficiently  close  inter- 
vals, suggested  to  Dr.  Heintz1  the  adoption  of  glass  mix- 
tures on  the  type  of  porcelain  glazes,  for  the  same  pur- 
pose. Their  cheapness,  the  possibility  of  forming  them 
into  masses  easily  observed  without  withdrawal  from  the 
place  of  exposure,  rendered  them  suitable  for  the  pur- 
pose. 

The  chief  problem  became  the  making  of  as  great  a 
range  of  them  as  possible  without  the  introduction  of 
reducible  elements,  and  to  make  the  composition  such  as 
to  obviate  all  danger  of  their  undergoing,  on  long  expos- 
ure, a  crystalline  change  or  devitrification  which  would 
alter  or  disturb  their  certain  melting  down,  the  condi- 
tion in  which  the  ultimate  value  of  a  large  part  of  the 
series  of  Prinsep's  alloys  failed. 

This  problem  was  undertaken  by  Seger,2  and  solved 


1  Thonindustrie-Zeitung,  7886,  p.  135. 

2  Thonindustrie-Zeitung,  1886,  pp.  135,  145,  168. 


PYROMETRY.  33 

so  successfully  that  his  "  Normal  Pyrometric  Cones"  an- 
swer all  conditions  required  in  ceramic  pyrometry. 

He  established  that  the  most  fusible  mixture  of  the 
porcelain  glaze  type  was  one  of  the  chemical  compo- 
sition : 


From  this  the  more  infusible  glasses  were  obtained  by 
the  systematic  increase  of  silica,  with  a  proportional  in- 
crease of  alumina  to  correct  the  well-known  tendency 
toward  devitrification  of  highly  siliceous  glasses. 

By  the  partial  substitution  of  alumina  with  ferric  oxid, 
the  series  was  brought  down,  in  its  member, 

o.3K20}o.3Al203}d  Si0 
o.yCaO  jo.2Fe203  I4       " 

to  equal  the  melting  point  of  the  alloy  ten  per  cent,  plati- 
num, ninety  per  cent,  gold,  and  thus  became  continuous 
with  the  most  useful  members  of  Prinsep's  series. 

The  use  of  the  cones  proved  to  be  so  practical  for  the 
purpose  that  the  need  to  substitute  the  Prinsep  alloys  by 
equivalent  cones  was  soon  felt,  and  -E.  Cramer,2  making 
use  of  the  well-known  fact  that  if  the  acidity  of  a  glass  be 
maintained,  but  the  silica  substituted  by  boracic  acid, 
the  melting-point  is  lowered,  increased  the  series  to  be- 
gin with  the  melting-point  of  silver. 

These  pyrometers  are,  in  the  entire  range  demanded 
by  ceramic  firings,  represented  in  the  following  list. 


1  Thouindustrie-Zeitung,  1886,  p.  168. 

2  Thonindustrie-Zeitung,  1892,  p.  155. 


34 


THK   CHEMISTRY   OF   POTTERY. 


For  the  benefit  of  those  who  find  it  difficult  to  free 
themselves  from  the  association  of  thermometric  degrees 
with  advances  in  heat,  such  degrees  have  been  interpo- 
lated, from  calorimetric  determinations,  as  follows  : 

Cone oio,  melting  down  with  pure  silver,  960° Celsius; 
cone  i,  melting  with  the  alloy  ninety  pet  cent,  gold,  ten 
per  cent,  platinum,  1 150°  Celsius ;  cone  20,  1700°  Celsius. 


Cone 

Estimated 
temperature 
in  degrees 

number. 

Chemical  Composition. 

Celsius. 

OIO       •] 

I      0.7CaO 

o.2Fe2O3 
o.3A!2O3 

3-5oSi02 
o.5oB2O3 

}        960 

09    \ 

r    o.3K2o 

L     o.7Cao 

o.2Fe2O3 
o.3A!2O3 

3-55810, 
o.45B203 

}       979 

08    < 

f     o.3K20 
L     o.7CaO 

o.2FeO, 
o.3Al263 

3.6oSiO2 
o.4oB2O3 

}       998 

j 

f      o.3K2O 

o.2Fe2Os 

3.65Si02 

V           T(~lT  *7 

07     < 

L     o.;CaO 

o.3A!203 

Q.35BA 

i        ALII  y 

06    < 

f     o.3K20 
L     o.7CaO 

.  o.2Fe.2O3 

o.3A!2O3 

3.7oSi02 
o.3oB2O3 

|    1036 

f     o.3K2O 

o.2Fe203 

3-75Si02 

I             Tf~\C  C 

°5     < 

t     o.7CaO 

o.3A!2O3 

0.258,0, 

|      IO55 

04     < 

r     o.3K20 
t     o.7CaO 

o.2Fe2O3 
o.3Al203 

3.8oSi02 
o.2oB2O3 

}      I074 

03     - 

f     o.3K20 
t     o.7CaO 

o.2Fe2O, 
o.3A!2O3 

o.isB2O3 

}      1093 

02       - 

f     o.3K20 
I.     o.7CaO 

o.2Fe.2O3 
o.3A!203 

o.ioB2O3 

I        1112 

01       < 

r   o.3K2o 

i.     o.7CaO 

o.2Fe203 
o.3A!203 

o.o5B2O3 

}        "SI 

I       < 

r   o.3K2o 

t     o.7CaO 

o.2Fe2O3 
o.3Al.2O3 

|     4SiO2 

|        1150 

2       - 

f     o.3K20 
(     o.7CaO 

o.iFe2O, 
o.4A!203 

}     4SiO2 

}        U79 

3       < 

f     o.3K20 
(     o.7CaO 

o.o5Fe2O3 
o.45A!2O3 

}     4SiO2 

|        1208 

4     • 

n  ^PaO 

{  o.5A!2O3 

}     4Si02 

}        1237 

PYROMKTRY.  35 

Estimated 
temperature 

Cone  in  degrees 

number.  Chemical  composition.  Celsius. 

5  {     ayCaO    {  °'5A12°3    }     5SiO2      }     1266 

6  {     C:7CaO    {  °-6A1*°3    }     6Si°2      }     I295 

' 


1323 
8 


'35* 
9     {     C^CaO   {  °-9A1203    }     9Si02      }     1381 

10  {     C.'yCaO   {  I'°A1'°3    }    IoSiO^      }     HIO 

11  {     ayCaS   {  X-2A12°3    }    I2«iO2      }     1439 

12  {     ayCaO   {  r^A1203    }   i4Si02     }     1468 


*  I497 


15  2'IA1*°3        2rSi  1555 

16 


18  {     al&O   {  3'IA12°3    }  3iSi02      }     1642 

19  {     ayCaO   {  3.5A1203    }  35SiO2      }     1671 

20  {     ayCaO   {  3-9Al2O3    }  39SiO2      }     1700^ 

In  order  to  secure  for  these  pyrometric  mixtures  a 
standard  character,   and    justify  the   name,    "  Normal 

1  Thonindustrie-Zeitung,  No.  49,  ffyj,  p.  1252-3. 


36  THE   CHEMISTRY  OF   POTTERY. 

Pyrometric  Cones,"  they  are  manufactured  by  the 
Prussian  Government  in  their  ceramic  experiment  sta- 
tion at  the  Royal  Porcelain  Works,  in  Charlottenburg, 
near  Berlin. 

It  is,  however,  not  at  all  difficult  to  make  them  of  suf- 
ficient accuracy,  from  the  excellent  native  materials  that 
are  on  the  market  in  the  United  States. 

Thus  the  original  series  of  Dr.  Seger  are  made  as  fol- 
lows :  The  potash  is  taken  in  the 
form  of  orthoclase ;  the  remaining 
alumina  that  is  requisite  is  introduced 
in  the  form  of  kaolinite ;  and  the 
remaining  silica,  not  supplied  by  these, 
is  added  as  quartz.  Calcium  carbon- 
ate gives  the  calcium  oxid,  and  those 
needing  ferric  oxid  have  it  directly 
added. 

The  necessary  amounts  of  these  in- 
gredients are  weighed  off  for  each  num- 
ber, introduced  with  water  into  a  small 
porcelain  jar  mill,  thoroughly  ground 
for  half  a  day,  settled,  the  water  drawn 
off,  and  the  mixture  dried.     It  is  then 
worked  into  a  mass  with  dextrin  muci- 
lage, and  formed  into  tetrahedrons  six 
NATURAL  SIZE.       centimeters  high  and  one  and  one-half 
centimeters  on  the  sides  of  the  triangular  base.1 

The  materials  selected  by  Dr.  Seger  as  sufficiently 
pure  for  the  purpose  analyzed  as  follows : 

1  Thonindustrie-Zeitung,  1886,  p.  136. 


A  SEGER  CONE  IN 


PYROMETRY. 


37 


Rorstrand 
feldspar. 
Per  cent. 

Silica 64.32 

Alnmina 19-41 

Ferric  oxid.-     0.14 

Lime trace 

Magnesia 0.35 

Potash 12.90! 

Soda 2.10  / 

Lossonglow'g   0.57 
Carbon  dioxid    


99-74 


Zettlitz 

kaolin. 

Per  cent. 

46.87 

38.56 

0.83 

trace 

trace 

.06 
12.73 


100.05 


Norwegian 

quartz. 

Per  cent. 

98.52 

1.04-1 
0.04  j 


0.40 


Carrara 
marble. 
Per  cent. 
1.00 


0.12 
54-93 

0.20 


43-76 


100.02 


One  can  make  with  the  raw  materials  produced  and 
put  on  the  market  in  this  country  quite  as  near  an  ap- 
proach to  the  theoretical  figures,  and  need,  therefore, 
have  no  hesitancy  in  using  them  for  such  a  purpose. 
The  following  analyses  will  verify  this  fact  : 

Kaolin,  from  Western  North  Carolina,  of  which  the 
writer  has  made  over  fifty  analyses,  not  varying  mate- 
rially from  the  following  extremes  : 

Analyzed 

November,        January, 
1894. 


1890. 

Silica 46.47 

Alumina 38.82 

Ferric  oxid 0.89 

Lime 0.28 

Magnesia 0.25 

Potash 0.63  -j 

Soda 0.12  } 

Combined   water °3-34 


100.53 


46.67 

38.14 

0.36 

0.50 

0.09 

0.64 
13.61 

100. I I 


Feldspar  from  New  York  : 


38  THE   CHEMISTRY   OF   POTTERY. 

Per  cent. 

Silica  65.85 

Alumina 19-32 

Ferric  oxid 0.24 

Lime 0.56 

Magnesia 0.08 

Alkalies1 14.10 


100.15 
Quartz  from  Illinois : 

Alumina o.  155 

Ferric  oxid 0.069 

Lime 0.026 

Magnesia 0.013 

Alkalies 0.112 

Total  impurities °-375 

Silica  (by  difference) 99-625 

100.000 

Commercial  Whiting : 

Silica trace 

Alumina      •» 

Ferric  oxid  / o>  *3 

Magnesia trace 

In  order  to  make  cone  No.  i,  having  the  formula 
o.3K2O)o.2Fe2O3) 

USi02 
o.7CaO)o.3Al203) 

there  would  be  ground  together  and  formed  into  tetra- 
hedrons, as  described, 

0.3  equivalents  feldspar 83.55  parts. 

0.7          "  calcium  carbonate  ..     35.00     " 

2.2  "  quartz 66.00     " 

0.2  "  ferric  oxid 16.00     " 


1  Combining  weight,  45.9. 


PYROMETRY.  39 


Xo.5Al,03.4Si0.2. 


For  cone  No.  4,  having  the  formula 
o.3K20 

o.yCaO 
there  would  be  used 

0.3  equivalents  feldspar 83.55 

0.7  calcium  carbonate- •  35.00 

0.2          '•  kaolinite 25-9° 

1.8          "  quartz 54.00 

For  cone  No.  10,  having  the  formula 

o.3K2CM 

ViAl2O3.ioSiO2. 
o.yCaO  J 

there  would  be  used 

0.3  equivalents  feldspar 83.55 

0.7           "             calcium  carbonate  -  35.00 

0.7                        kaolinite 90.65 

6.8          "            quartz 204.00 

For  the  introduction  of  boracic  acid  into  the  lower 
scale,  according  to  E.  Cramer  this  element  is  first  fixed 
as  a  lime-soda  glass  of  the  composition 

o.5Na2O)  (2SiO2. 

to.iAl.Qj 

o.sCaO   J  liB203. 

by  melting  together 

191  parts  crystallized  borax. 
50     "•     calcium  carbonate. 
52      "       kaolinite. 
96      "       quartz. 

Then  equivalent  amounts  of  this  glass  are  ground  with 
equivalent  amounts  of  feldspar,  kaolinite,  quarz,  calcium 
carbonate,  and  ferric  oxid,  to  make  bodies  of  the  corres- 


40  THK    CHEMISTRY   OF   POTTERY. 

ponding  formulas,  and  the  masses  are  formed  into  tetra- 
hedrons, as  described  before. 

The  use  of  the  Seger  cones  is  very  simple.  One  of 
the  number,  representing  the  heat  which  it  is  wished  to 
reach,  is  set  upright  in  each  of  the  proper  places  of  the 
kiln  opposite  a  spy-hole,  which  latter  is  closed  with  a  plate 
of  mica.  When,  by  the  advancing  heat,  the  cone 
bends  until,  finally,  its  apex  touches  the  base  on 
which  it  is  set,  the  temperature  which  it  is  intended  to 
indicate  is  reached,  and  the  firing  stopped. 

The  cones  should  be  set  about  three  feet  back  from 
the  kiln-wall,  in  a  piece  of  fire-clay  piping  or  muffle  ex- 
tending to  the  kiln-wall  and  running  a  foot  or  more 
back  of  the  pyrometer,  so  that  this  is  seen,  when  the  fire 
is  well  advanced,  as  a  luminous  body  in  a  darker  field. 
The  pyrometer  is  best  stuck  with  a  little  soft  mud  on  a 
fire-clay  shard  or  tile,  to  prevent  its  falling  over. 

The  determination  of  the  proper  pyrometer  to  be  used 
as  the  index  of  the  heat  to  wThich  a  clay  under  examina- 
tion should  be  burned  is  best  explained  in  connection 
with  the  examination  of  clays  for  specific  industries,  and 
is  deferred  to  these  chapters. 


CHAPTER  IV. 
CLASSIFICATION  OF  CERAMICS. 


GENERAL  classification  of  clay  manufac- 
tures will  be  of  service  in  this  connection, 
particularly  to  the  chemist,  who  may  not, 
heretofore,  have  given  much  considera- 
tion to  ceramics,  in  order  to  emphasize 
characteristics  that  are  of  moment  to  the 
potter. 

The  varieties  of  ware  manufactured  in  the  United 
States  are  not  very  great,  and  can  be  quite  sharply  char- 
acterized. It  will  therefore  be  most  practical  to  take  as 
simple  a  classification  as  possible,  especially  as  it  is 
mainly  made  for  the  benefit  of  the  technical  chemist, 
and  not  for  the  connoisseur,  and  need  merely  enumerate 
the  chief  technical  characteristics  without  classifying  all 
their  possible  combinations. 

The  old  classification  of  Brogniart  is  accordingly 
taken  as  a  basis  for  the  purpose. 

He  divides  all  ceramic  products  into  three  main 
classes  and  nine  subdivisions,  as  follows  : 

FIRST  Civ  ASS. — Bodies  sufficiently  soft  to  be  scratched 
with  a  knife,  of  a  sandy-argillaceous  character  or  con- 
taining lime  and  fusible  in  the  heat  of  the  porcelain  kiln. 
First  Division. — Bodies  soft  burned,  with  a  dull  un- 
glazed  surface.  Examples  :  Brick,  building  terra-cotta, 
drain  tile,  etc. 


42  THE   CHEMISTRY   OF   POTTERY. 

Second  Division. — With  the  faint  luster  of  an  alkali- 
earthy-silicate,  the  gloss  produced  by  polishing  or  by 
incipient  fusion.  Example  :  Antique  vessels. 

Third  Division — A  body  similar  to  that  of  the  first  or 
second,  but  with  a  transparent  lead  glaze.  Example  : 
Common  pottery  ware. 

Fourth  Division. — Ware  enameled  with  nontransparent 
glazes  containing  tin  oxid.  Examples  :  Tile  for  porce- 
lain stoves,  and  common  dishes. 

SECOND  CLASS. — Bodies  that  are  hard,  nontransparent, 
of  a  siliceous-argillaceous  mass  that  cannot  be  scratched 
with  a  knife,  and  are  infusible. 

Fifth  Division. — White  body  with  a  transparent  lead 
glaze.  Examples:  Fine  faience  and  dish- ware. 

Sixth  Division. — A  colored  body  with  an  earthy  alka- 
line silicate  glaze,  or  sufficiently  dense  to  require  no 
glaze.  Example :  Stoneware. 

THIRD  CLASS. — Bodies  hard,  translucent,  high*  in 
alkali,  siliceous-argillaceous,  softening  in  the  hardest 
fire. 

Seventh  Division. — Kaolinitic  body  with  a  glaze  mainly 
feldspathic.  Example:  True  hard  porcelain. 

Eighth  Division. — Body  of  kaolin,  plastic  clay,  and 
bone  ash,  with  a  lead-boracic-acid  glaze.  Example  : 
English  soft  porcelain. 

Ninth  Division. — Body  of  a  glass-frit  with  addition  of 
clay  and  a  lead  glaze.  Example :  French  soft  porce- 
lain. 

In  addition  to  the  manufactures  innumerated  by  Brog- 
niart  in  his  first  division,  flooring  tile  would  belong  in 


CLASSIFICATION   OF   CERAMICS.  43 

this  category.  It  is  now,  however,  customary  to  burn 
these,  as  well  as  the  better  grades  of  building  terra-cot- 
tas,  so  hard  that  they  are  not  to  be  scratched  with  a 
knife  ;  enabling  them  to  better  resist  wear  and  the  dis- 
integrating action  of  the  elements. 

Our  common  "  red  ware"  corresponds  exactly  to  that 
of  the  third  division. 

In  the  fourth  division,  the  only  representatives  of 
American  ware  are  white  enameled  brick.  Occasionally 
dishes  are  brought  to  the  New  England  coast  towns 
from  Fayal,  in  the  Azores,  corresponding  exactly  to 
those  described  by  Brogniart ;  but  clays  rich  in  lime, 
such  as  the  tin  enamels  melt  best  on,  are  only  used  with 
us  in  the  manufacture  of  cream-colored  brick — not  for 
dishes  ;  and  tin-enamels,  apart  from  their  employment 
on  brick,  have  only  been  employed  on  metal  vessels. 

Our  common  dishes,  better  in  grade  than  red  ware, 
are  made  in  yellow  and  Rockingham  ware,  which  should 
come  under  the  second  class,  in  having  a  hard  siliceous- 
argillaceous  body  that  cannot  be  cut  with  a  knife,  and  is 
porous  and  opaque.  There  is  no  division,  however,  into 
which  it  will  exactly  fit,  for  the  body  is  yellow  and  the 
glaze  transparent  and  plumbiferous,  and,  in  the  case  of 
Rockingham  ware,  manganiferous. 

All  enumerated  varieties  of  ware  of  the  fifth  division 
are  common  with  us,  and  are  variously  termed  cream- 
colored  ware  ( "  C.  C. " ) ,  white  granite  ware  ( ' '  W.  G. "  ) ' 
ivory  white  ware,  ironstone  china,  etc. 

Similarly,  all  classes  of  the  sixth  division  are  common 
in  the  United  States,  the  stoneware  being  commonly 


44  THE    CHEMISTRY   OF   POTTERY. 

divided  into  "salt-glazed"  and  "slip-glazed"  products, 
the  meaning  of  which  terms  will  be  explained  in  the 
chapter  devoted  to  these  wares. 

The  paving-brick  now  extensively  manufactured  in 
the  United  States  come  under  the  unglazed  products  of 
this  category. 

The  wares  thus  far  enumerated  are  called  in  English, 
collectively,  "pottery,"  and  it  is  with  the  chemical 
character  of  these,  particularly  with  the  glazed  varie- 
ties, that  this  book,  in  the  main,  concerns  itself. 

Those  coming  under  the  third  class  have,  within 
recent  years,  begun  to  be  manufactured  in  quantities 
assuming  proportions  of  commercial  importance,  with 
the  exception  of  the  French  pate  tendre,  which  is  not 
made. 

We  have,  however,  a  kind  of  ware  standing  between 
that  of  the  fifth  division  and  porcelain,  for  which  there 
is  in  this  classification  no  exact  place.  It  is  commonly 
known  as  "hotel  china,"  and  resembles  porcelain  in 
having  a  vitreous  body,  rich  in  alkali,  that  in  thin  places 
would  be  translucent,  but  which  is  covered,  like  the 
ware  of  the  fifth  division,  with  a  clear  lead  glaze. 

There  is  one  misnomer  in  popular  use  that  may  cause 
the  chemist  looking  up  the  proper  significance  of  the 
term  some  misunderstanding;  namely,  the  word  "  Ma- 
jolica. ' '  In  the  United  States,  it  is  commercially  a  cheap 
ornamental  ware,  with  a  white,  rather  soft  body,  deco- 
rated with  designs  in  relief,  and  these  designs  tinted 
with  soft,  transparent,  colored  glazes. 

Originally,   the  term  was  applied  by  the  Italians  of 


CLASSIFICATION   OF   CERAMICS.  45 

the  fifteenth  and  sixteenth  century  to  white  enameled 
ware  decorated  in  lusters — that  is,  in  colors  having  a 
metallic  reflex — which  was,  at  first,  of  Moorish  make, 
and  brought  to  Italy,  largely  from  the  island  of  Majorca 
and  afterward  it  was  applied  to  all  ornamented  white 
enameled  ware  ;  ware  belonging,  however,  in  all  cases, 
to  the  fourth  division  of  Brogniart. 

The  word  faience  is  also  one  commonly  misunder- 
stood. Originally,  it  meant  majolica  from  the  potteries 
of  Faenza,  but  finally  became  synonymous  with  majolica 
in  general,  meaning  an  ornamented  white  ware.  But 
the  name  was  most  commonly  used  in  France,  where 
white  ware,  made  with  white  clays  and  a  transparent 
plumbiferous  glaze,  largely  took  the  place,  both  in  orna- 
mental and  useful  articles,  of  the  white  tin-enameled 
pottery  ;  so  that  the  term  lost  its  specific  meaning,  and 
came  to  mean  pottery  of  a  little  better  grade  in  general. 

Ceramists  are  inclined  now  to  use  the  term  "  faience" 
in  distinction  from  the  application  of  the  word ' '  majolica, ' ' 
using  it  to  designate  potteries  with  transparent  glazes, 
and  majolica,  as  a  generic  term  for  potteries  with  intrans- 
parent  glazes. 

In  this  sense,  our  potters  are  already  using  the  word 
faience  quite  correctly  for  ornamented  wares  belonging 
both  to  the  third  and  fifth  division  of  Brogniart. 

It  is  to  be  hoped  that  our  incorrect  application  of  the 
word  majolica,  which  is  only  used  as  a  trade  name  for 
an  inferior  grade  of  ornamental  ware,  will  sink  with  the 
latter  into  obloquy. 

The  word   "enamel"  has  attained  a  specific  signifi- 


46  THE   CHEMISTRY   OF   POTTERY. 

cance  which  the  public  frequently  misapplies,  using  it 
indiscriminately  for  all  glazes  or  glassy  coverings. 

Technically,  the  word  enamel  means  an  nontransparent 
glass,  completely  covering  the  body  upon  which  it  is 
melted,  the  nontransparency  being  due  to  the  presence  of 
tin  oxid,  arsenic,  bone  ash,  or  cryolite. 

It  is  important  that  the  technical  significance  be  ad- 
hered to,  and  that  the  indiscriminate  popular  use  of  the 
word  be  realized,  to  prevent  misunderstandings. 

Ceramic  wares  pass  into  each  other  by  almost  imper- 
ceptible gradations.  The  great  variety  of  possible 
bodies,  glazes,  decorations,  means  of  shaping,  applica- 
tions, opens  an  almost  infinite  field  to  the  fancy  of  the 
worker  and  the  greatest  number  of  combinations  to  meet 
specific  uses. 

It  is  an  error  to  suppose  that  the  so-called  finer  grades 
of  ceramics,  though  made  of  pure  kaolin,  feldspar,  and 
quartz,  and  white-burning  secondary  clays,  alone  offer 
artistic  possibilities  or  problems  worthy  of  the  chemist's 
consideration. 

On  the  contrary,  the  wares  that  have  brought  us  the 
interest  of  foreign  countries  belong  to  the  first,  third,  and 
sixth  division  of  Brogniart's  classification;  they  repre- 
sent the  largest  commercial  factor  in  our  clay  industries, 
and  in  their  midst  the  greatest  artistic  success  has  been 
achieved. 

While  we  are  better  supplied  with  pure  pottery  mater- 
ials than  any  other  country,  our  common  clays  will,  and 
properly  should,  always  claim  the  greatest  share  of 
interest,  not  only  because  they  must  be  the  raw  mate- 


CLASSIFICATION   OF    CERAMICS. 


47 


rials  of  the  bulkiest  manufactures,  and  those  in  which  the 
greatest  economies  will  have  to  be  exercised,  but  be- 
cause they  will  always  offer  the  greatest  technical  and 
artistic  possibilities. 


CHAPTER  V. 
POTTERY  GLAZES. 


S  EXPLAINED  in  the  previous  chapter,  pot- 
tery bodies  have,  in  the  main,  an  earthy 
fracture,  and  even  when  sufficiently  hard  so 
as  not  to  be  scratched  with  a  steel  point,  may 
absorb  fifteen  to  twenty  per  cent,  of  their  vol- 
ume of  liquid,  with  avidity.  But  even  vitre- 
ous bodies  that  were  very  brittle,  and  had  a  conchoidal 
glassy  fracture,  were  found  by  the  writer  to  absorb  up  to 
one  and  eight-tenths  per  cent,  of  their  weight,  or  nearly 
four  per  cent,  of  their  bulk,  of  distilled  water. 

From  this  it  will  be  seen  that  all  pottery  and  porcelain 
articles,  if  they  are  to  serve  as  containers  or  be  subject 
to  conditions  making  frequent  cleaning  necessary,  should 
be  covered  with  an  impervious  surface  coating.  This  is 
practically  always  a  glass. 

The  glass,  or  "glaze,"  as  it  is  designated,  must  neces- 
sarily answer  the  following  conditions  : 

1 i )  It  must  have  practically  the  same  coefficient  of  ex- 
pansion as  the  body  of  the  ware,  in  order  not  to  "  craze"  ; 
that  is,  tear  with  transverse  cracks ;  or  not  to  push  off  on 
high  places  and  sharp  angles,  tearing  the  body  along 
with  it,  called  "  shivering." 

(2)  It  must  bear  exposure  in  thin  layers  to  gradually 


POTTERY   GLAZES.  49 

increasing  temperatures,   lasting  hours,  or  even  days, 
during  baking,  without  suffering  devitrification. 

(3)  It  must  be  sufficiently  tenaceous  when  melted  so 
as  not,  in  the  necessarily  long  time  in  which  it  is  in  this 
state,  to  run  off  of  upright  objects,  nor  yet  so  stiff  as  not 
to  flow  perfectly  smooth  on  flat  ones. 

(4)  It  must  hold  dissolved,  without  unsightly  separa- 
tions, metallic  oxids  that  have  been  added  for  imparting 
color. 

(5)  It  must  not  exert  too  strong  a  solvent  action  upon 
oxids  used  in  painting  and  printing  upon  the  body  of 
the  ware,  over  which  the  glass  is  afterward  melted  like 
a  covering  of  indestructible  varnish. 

The  difficulties  attending  the  fulfilling  of  these  general 
conditions,  besides  many  special  ones  that  occur  in  con- 
nection with  every  kind  of  ware,  have  naturally  led  pot- 
ters to  guard  the  composition  of  those  that  answered 
their  particular  purposes  with  great  jealousy. 

The  formulas  that  have  been  published  are,  in  the 
main,  misleading,  or,  where  this  is  not  the  case,  they 
are,  as  a  rule,  equally  useless,  inasmuch  as  the  inter- 
dependence of  glaze,  clay,  and  the  condition  of  firing  is 
so  intimate  that  even  a  correct  description  of  one,  with- 
out the  others  in  conjunction,  is  of  no  practical  service. 

It  is  furthermore  impossible  to  judge  a  priori  of  what 
may  be  expected  of  a  glaze  in  melting-point,  coefficient 
of  expansion,  and  tendency  toward  devitrification  from 
its  gross  composition  by  weight.  This  must  be  resolved 
into  the  chemical  formula,  and  in  order  to  do  this,  the 
chemical  character  of  the  compound  and  the  function  of 
its  elements  must  first  be  understood. 


50  THE   CHEMISTRY   OF   POTTERY. 

The  practical  experience  of  the  potter  teaches  him  that 
he  needs  as  a  glaze  a  body  resembling  in  hardness,  color, 
and  refracting  power  ordinary  flint  glass ;  but  that  ground 
flint  glass,  painted  on  his  ware  and  exposed  in  the  kiln, 
will  not,  under  any  circumstance,  give  him  a  glaze  having 
the  qualities  the  glass  before,  in  itself,  had,  but  a  puck- 
ered, de vitrified  mass,  barely  sintered  together. 

To  whatever  circumstance  this  may  be  due,  he  knows 
still  that  what  he  needs  is  a  glass  or  a  mixture  composed 
of  the  elements  of  a  glass,  which,  to  meet  his  special  pur- 
poses, can  be  modified  in  the  following  ways. 

To  reduce  the  melting  point,  the  alkalies,  alkaline 
earths,  and  certain  oxides  of  the  heavy  metals,  particu- 
larly and  almost  indispensably  lead,  must  be  increased ; 
that  with  this  increase  and  resulting  greater  fusibility, 
the  glaze  becomes  more  brilliant ;  that  is,  attains  a  higher 
refractive  index ;  but  will  inevitably  craze  if  the  increase 
of  these  bases  exceeds  a  certain  amount. 

Conversely,  to  counteract  the  crazing  of  the  more  fusi- 
ble glazes  on  certain  clays,  the  addition  of  quartz  to  the 
glaze  corrects  this  defect,  with  attendantly  growing  diffi- 
cult fusibility,  until,  finally,  the  coefficient  of  expansion 
is  so  far  changed  that  the  glaze  tends  to  push  off  from 
the  body  at  elevated  places  and  angles,  and  this  increase 
of  quartz  may  be  pushed  to  the  extent  that  the  glaze  will 
shatter  the  entire  piece  of  ware  which,  as  already  said, 
is  called  ' '  shivering. ' '  These  are  the  extremes  between 
which  the  proportion  of  base  and  acid  must  lie. 

The  action  of  boracic  acid  is  similar  to  that  of  the 
quartz  or  silica  in  changing  the  coefficient  of  expansion, 


POTTBRY  GLAZES.  5^ 

in  opposition  to  the  effect  of  the  bases,  but  the  melting- 
point  does  not  rise  in  the  .same  proportion.  On  the  con- 
trary, by  the  substitution  of  silica  with  boracic  acid,  the 
melting  point  falls  very  materially.  The  glaze  also  in- 
creases in  brilliancy  with  this  addition,  but  is  softer, 
being  more  easily  scratched,  and  exerts  a  far  greater  sol- 
vent action  on  under-glaze  colors. 

Glazes  composed  of  the  above  constituents,  while  they 
seem  to  answer  every  requirement  when  melted  in  the 
experimental  muffle,  fail  utterly  when  used  in  the  large 
ware  kiln.  The  potter  has  found  empirically  that  no 
glaze  can  be  perfect  without  the  addition  in  certain  pro- 
portion, of  either  clay,  feldspar,  or  Cornwall  stone,  and 
the  chemist  will  readily  recognize  that  the  new  element 
introduced  by  these  materials  is  merely  alumina,  and  that 
it  is  this  and  other  chemically  similar  elements  which 
keep  the  glaze  from  devitrification  in  the  protracted 
'glost-fire"  of  the  potter's  oven. 

In  calculating  the  chemical  formula  of  glazes,  the  ba- 
ses, with  the  exception  of  those  of  the  alumina  group, 
are  reduced  to  their  equivalent  proportions,  and  multi- 
plied by  such  a  factor  that  their  sum  equals  unity.  The 
corresponding  equivalent  weights  of  the  members  of  the 
alumina  group  are  then  recorded  together,  as  they  oc- 
cupy an  intermediary  position,  in  fixing  the  relations  of 
the  bases  to  the  acids,  under  protracted  firing,  and  then 
the  equivalent  weights  of  the  acids  are  noted. 

A  glaze  would  then  have  the  formula  icROx.  R2O3y. 
SiO2. 

A  common  flint  bottle-glass  was  found  to  have  the 
composition : 


52  THE   CHEMISTRY   OF   POTTERY. 

Soda  ..............................  17.80  per  cent. 

Lime  .............................  6.37     "       " 

Magnesia  .........................  2.83     "       " 

Alumina  ..........................  0.53     "       " 

Silica  (by  difference)  ..............  72.47     "       " 

100.00 
the  chemical  formula  being 

0.61  Na.2O  | 
0.24  CaO   f  2.56S1O,. 
^ 


0.15 
i.o  RO. 

Finely  ground  and  applied  to  a  burnt  shard  of  clay,  it 
was  scarcely  possible  to  get  with  it  a  bright  transparent 
glaze  in  the  experimental  muffle. 

By  taking  a  half  equivalent  of  the  glass  and  one-half 
equivalent  of  a  lead  compound,  and  making  up  the  silica 
by  addition  of  ground  flint,  as  follows  : 

r  0.305  Na2O  i 
0.5  equivalent  of  the  glass  •!  0.120  CaO     [•  1.28  SiO2 

1  0.075  MgO  J 
t  0.5      PbO       1.28  SiO2 

i.o  2.56 

• 

made  by  thoroughly  mixing  in  a  mortar  with  water  52.71 
flint  glass,  55.75  litharge,  and  38.4  flint  ;  and  painting 
the  mixture  on  a  potsherd,  it  flowed  to  a  brilliant  glaze 
at  a  temperature  above  that  of  the  melting  point  of  an 
alloy,  fifty  per  cent,  silver  and  fifty  per  cent.  gold. 

In  the  potter's  glost-kiln,  under  a  thirty  hours'  fire 
and  two  days'  cooling,  the  glaze  was  puckered  and  dull. 


POTTERY   GLAZKS.  53 

The  introduction  of  two-tenths  equivalents  of  alumina, 
in  the  form  of  china  clay,  with  allowance  for  the  silica 
necessarily  introduced  with  it,  corrected  it  for  these  con- 
ditions so  that  the  glass,  composed  of 

0.5    equivalent  flint  glass,  52.71  parts, 

0.5  "  litharge,     55.75     " 

0.2  "  china  clay,  25.9       " 

0.88  "  flint,  26.4      " 

answered  on  a  shard  of  the  proper  coefficient  of  expan- 
sion, and  came  brilliant  and  clear  from  the  long  fire. 

This  example,  illustrating  the  functions  of  the  differ- 
ent elements  composing  a  glaze,  sufficiently  explains  the 
structure  of  the  chemical  formula  and  the  insight  it  gives 
one  into  a  complicated  glaze  formula. 

It  must  not  be  inferred  from  the  above,  however,  that  a 
flint  glass  should  be  the  basis  of  every  pottery  glaze,  nor 
that  every  glaze  must  contain  lead. 

Potters  are  accustomed  to  distinguish  between  "raw" 
and  "fritted"  glazes.  The  former  have  merely  their 
constituents  ground  together  and  are  melted  to  a  glass 
for  the  first  time  when  exposed  on  the  ware  to  the  glost- 
fire. 

Fritted  glazes  are  those  that  contain  a  glass  or,  rarely, 
are  such  as  have  been  entirely  melted  to  a  glass  before 
grinding  and  putting  on  the  ware. 

The  purpose  of  fritting  all  or  a  part  of  the  constituents 
of  a  glaze  is  primarily  to  render  insoluble  such  constitu- 
ents as  would,  by  their  solution  in  water,  be  carried  away 
by  the  customary  wet-grinding  and  the  application  of 
the  glaze,  suspended  in  water,  to  the  ware.  This  is  the 


54  THE   CHEMISTRY   OF   POTTERY. 

case  when  soda-ash,  pearl-ash,  borax,  niter,  boracic  acid, 
and  similar  constituents  are  to  be  incorporated  in  the  glaze . 
At  times,  too,  when  a  glaze  contains  large  amounts  of 
alkaline  earths,  with  little  or  no  lead  oxid,  fritting  is 
necessary,  as  the  alkaline  earths,  not  being  in  themselves 
fusible,  enter  with  difficulty  into  combination,  and  would 
require,  for  the  incipient  union,  heats  far  too  excessive  for 
the  glass  when  actually  formed. 

In  order  to  illustrate  the  calculation  of  the  formula  of 
a  glaze  from  its  analysis,  the  following  example  will  serve. 
The  sample  was  taken  from  a  plain-glazed  tile  of  Amer- 
ican manufacture,  as  explained  in  the  first  chapter,  and 
gave  the  following  percentage  composition : 

Lead  oxid 24.21  per  cent. 

Alumina 11.58  "  " 

Silica 49.72  "  " 

Lime 0.86  "  " 

Magnesia 0.73  "  " 

Potash 0.42  " 

Soda 4-68  "  " 

Sulfuric  anhydrid 1.46  "  " 

Boracic  acid  (by  difference) 6.34  "  " 

100.00 
The  chemical  formula  of  the  glaze,  then,  is — 

0.4952  PbO    %  r3.78oSiOa 

0.1296  CaO    [0.5728  A12O3  J 

0.3752  Na2O  )  1 0.413  BO3 

i.o        RO 

In  order  to  make  this  glaze,  one  would  melt  together 
to  form  a  fritt — 


POTTERY   GLAZES.  55 

Parts  give          Melted. 

0.207   equivalent  borax 39.5  borax  glass  20.91 

0.168  "  soda  ash...     8.9  Na.X)  5.21 

0.130  "  whiting 6.5  CaO  3.64 

o.ioo            "           china  clay  .   13.0  Al.2O32SiO2    11.15 
0.800  "  flint 24.0  SiO2  24.00 

The  charge  91.9  melts  to. . .  64.91 

For  the  glaze,  grind  together — 

0.4952  equivalent  white  lead 64.23 

0.4128          "  china  clay 53-46 

2.78     equivalents  flint 83.4 

fritt 64.91 

266.00 

It  will  be  noted  that  the  analysis  contains  an  apprecia- 
ble amount  of  sulfuric  anhydrid,  which  was  ignored  in 
the  calculation  of  the  analysis. 

The  anatytical  sample  was  taken  from  the  edges  of  the 
tile,  where  the  glaze  was  thickest ;  but  here,  although 
not  over  the  surface  of  the  glaze  generally,  there  was  a 
crystalline  scum.  This  was  "  glass-gall,"  or  a  crystal- 
line separation  of  sulfates.  The  proportion  of  sulfuric 
anhydrid  was  therefore  larger  than  if  the  sample  had 
been  averaged  over  the  entire  face. 

Sulfates  are  highly  objectionable  in  glazes.  They  may 
form  either  unsightly  crystalline  separations  on  the  edges 
of  the  ware,  or  cover  its  entire  face  with  a  very  thin 
greasy  scum,  and  when  present  in  large  amounts,  may 
float  in  drops  on  the  glaze,  from  which,  on  cooling,  they 
may  easily  be  pinched  out,  leaving  pits  in  its  surface. 

The  chemist  must  make  certain  that  the  materials  are 


56  THE   CHEMISTRY  OF    POTTERY. 

free  from  sulfates,  and  even  look,  at  times,  to  the  water 
in  which  the  glazes  are  ground. 

Sulfates  are  frequently  formed  by  the  absorption  of 
sulfuric  anhydrid  generated  in  the  kiln  from  the  burning 
of  a  very  sulfurous  coal.  Obviating  difficulties  that  may 
arise  from  this  will  be  discussed  under  the  subject  of 
firing.  It  may  merely  be  noted  here  that  the  phenome- 
non, commonly  called  "sulfuring"  by  potters,  is  the  re- 
duction of  the  lead  of  the  glaze,  and  is  often  the  catch- 
word with  which  a  careless  fireman  tries  to  throw  on  the 
fuel,  the  responsibility  for  inattention  to  clinkering  or 
other  manipulations  affecting  the  draught  in  firing. 

The  phenomenon  of  a  reduction  of  the  glaze  frequently 
hangs  together,  in  so  far,  with  the  presence  of  a  large 
amount  of  sulfur,  in  that  the  coal  contains  the  latter, 
mainly  in  the  form  of  iron  pyrites,  which,  leaving  on 
burning  the  iron  oxids,  give  a  more  or  less  fusible  ash, 
or  clinker,  which  is  very  liable  to  choke  off  a  part  of  the 
necessary  air  supply. 

The  range  of  formulas  of  pottery  and  porcelain  glazes 
is  from — 

i  RO  o.i  R2O3 1.5  SiO2 
to    i  RO  1.2  R2O3 12  SiO2> 

and  the  corresponding  range  of  temperatures  required  in 
their  burning  is  from  the  melting  point  of  silver,  or  pyr- 
ometric  cone  oio  of  Cramer's  scale,  to  pyrometric  cone 
1 8  of  Seger's  scale,  probably  equivalent  to  the  melting 
point  of  an  alloy — eighty  per  cent,  platinum,  and  twenty 
per  cent.  gold. 

In  this  wide  range  of  possible  compounds,  the  one  suit- 


POTTERY   GRAZES.  57 

ing  the  particular  conditions  demanded  by  the  clay  used, 
and  the  process  of  manufacture,  and  the  uses  to  which 
the  ware  is  to  be  put,  must  be  sought.  For  the  potter  to 
do  this  empirically  is  certainly  a  very  difficult  and  un- 
certain undertaking ;  for  the  chemist,  beginning  with  a 
mixture  based  on  a  definite  chemical  formula  within  the 
given  limits,  and  systematically  introducing  the  various 
possible  elements  in  proportions  varying  by  definite 
equivalents,  it  is  but  a  question  of  time  to  get  the  desired 
formula,  which,  with  experience  and  skill  in  experi- 
menting, need  not  take  long. 


CHAPTER  VI. 
RED  WARE. 

I  HE  simplest  and  cheapest  of  glazed  pottery  is 
called  from  its  color,  "  red  ware."  It  is  or- 
dinarily formed  from  the  same  materials  used 
for  making  red  brick;  alluvial  mud  found 
in  the  river  valleys  and  weathered  ferruginous  shales.  It 
is  important  that  the  material  contain  but  little  lime, 
which  if  present  in  any  considerable  amount,  destroys 
the  bright  red  color  imparted  by  the  iron  oxid,  giving 
unsightly  ware. 

At  a  heat  sufficient  to  melt  a  wire  of  pure  silver  to  a 
bead,  the  clay  should  bake  so  hard  that  it  can  barely  be 
cut  with  a  knife.  The  specimen  baked  at  that  heat 
should  adhere  when  touched  to  the  tongue,  and  be  of  a 
bright  red  color. 

The  clay  must  be  of  such  composition  that  with  the 
hardness  attained  at  the  given  temperature  (from  the 
melting  point  of  silver  to  not  exceeding  that  of  an  alloy 
of  seventy-five  per  cent,  silver,  twenty-five  per  cent,  gold) , 
it  will  have  practically  the  same  coefficient  of  expansion, 
and  hence  bear  without  fracture,  a  glass  fusible  at  that 
heat. 

A  glass  or  ' '  glaze ' '  of  this  character  would  be  one 
having  a  chemical  formula  lying  between  iPbO,  o.i 
A12O3,  iSiO2  and  iPbO,  o.i5Al2O3,  i.5SiO2  made  by 


RED  WARE.  59 

grinding  together  129.7  parts  white  lead,  13.0  to  19.5 
parts  china  clay,  and  24  to  21  parts  quartz. 

In  order  to  absolutely  resist  the  action  of  acids  on  cook- 
ing utensils  of  "red  ware,"  it  would  be  desirable  to  use 
more  acid  glazes,  but  such  is  not  the  practice  in  this  in- 
dustry ;  although  there  is  a  practice  that  should  be  abso- 
lutely condemned  and  against  which  chemists  should 
throw  their  influence,  namely,  that  of  using  litharge  or 
galena  alone  as  the  glazing  substance  and  depending  on 
its  taking  up  sufficient  alumina  and  silica  from  the  body 
of  the  ware,  during  fire,  to  form  the  glass.  A  glaze 
so  formed  is  certain  to  be  basic  on  the  surface,  and  is 
sure  to  be  attacked  by  the  weakest  acids  used  in  cookery. 

The  red  ware  potter  has  no  china  clay  at  his  disposal, 
and  usually  makes  his  glaze  by  grinding  the  lead  prepara- 
tion with  a  loamy  sand. 

The  glaze  then  has  frequently  some  such  composition 
as  this — 


(taken  from  practice).  This  glaze  would  be  of  a  yel- 
lowish color,  which  is  not  objectionable. 

The  coefficient  of  expansion  of  the  clay  depends  on 
the  amount  and  fineness  of  the  uncombined  silica  and 
feldspathic  detritus  it  contains,  which  constituents  are 
determined  by  the  "rational  analysis." 

A  practical  "red ware"  clay,  burning  at  the  indicated 
temperature  to  the  required  hardness  and  color,  and 
bearing  the  glaze  without  suffering  fracture  ( '  'shivering' ' ) 
or  the  glaze  itself  cracking  ("crazing"),  being  of  suffi- 


60  THE   CHEMISTRY   OF   POTTERY. 

cient  plasticity  and  having  from  the  clay  to  the  baked 
condition  a  linear  shrinkage  of  six  per  cent.,  is  a  weath- 
ered shale  of  the  following  analysis : 
TOTAI,  ANALYSIS. 

In  sol.  in 
Per  cent.        H2SO4  and  Na2CO3. 

Silica 74.75  57.20 

Alumina 12.55  0-62 

Ferric  oxid 5.28  0.70 

Lime 1.28  0.77 

Magnesia 0.85  o.oo 

Alkalies 2.27  1.80 

Combined  water 3.23 

100.21  61.09 

RATIONAL  ANALYSIS. 

Per  cent. 

Clay  substance 39.12 

Quartz 52.54 

Feldspathic  detritus 8.55 

PERCENTAGE  COMPOSITION  OF  THE  "  CI<AY  SUBSTANCE." 

Per  cent. 

Silica .  44.86 

Alumina 30.50 

Ferric  oxid 11.71 

Lime i  .30 

Magnesia 2.17 

Alkalies 1.20 

Combined  water 8.26 


100,00 

The  following,  a  highly  plastic  red  colored  clay, 
proved  unsuitable.  It  was  too  plastic  and  would  twist 
and  crack  in  the  fire ;  it  burned  to  a  dark  brownish  red, 
instead  of  a  light  bright  color,  shrinking  twelve  per  cent, 
in  the  fire.  It  contains  insufficient  uncombined  silica  to 


RKD    WARE.  6l 

bear  a  glaze  suited  to  the  red  ware  fire,  without  the  lat- 
ter's  "  crazing." 

TOTAL  ANALYSIS. 

Insol.  in 

Per  cent.  HaSO4  and  Na2CO9 

Silica 61.93  28.98 

Alumina 19.87  0.53 

Ferric  oxid 7.83  1.57 

Lime 1.61  0.16 

Magnesia 0.77  «.o8 

Alkalies 2.38  1.08 

Combined  water 5.91 


100.30  32.40 

RATIONAL  ANALYSIS. 

Per  cent. 

Clay  substance 67.90 

Quartz 21.57 

Feldspathic  detritus 10.83 

PERCENTAGE  COMPOSITION  OF  THE  "CLAY  SUBSTANCE." 

Per  cent. 

Silica 48.53 

Alumina 28.48 

Ferric  oxid 9.22 

Lime 2.14 

Magnesia  1 .02 

Alkalies 1 .92 

Combined  water 8.70 

PRODUCTS. — The  wares  manufactured  in  this  industry 
are  flower-pots  and  other  unglazed  terra-cotta  articles ; 
of  glazed  articles,  brown  door-knobs,  milk  crocks,  bean 
pots,  and  other  cooking  vessels,  pots  for  corroding  white 
lead,  jardiniers,  umbrella-stands,  spittoons,  etc.,  which 
latter  ornamental  pieces  are  often  decorated  on  the  outer 
unglazed  surface  with  oil  colors. 


62  THE   CHEMISTRY   OF   POTTERY. 

With  the  exception  of  these  crudely  ornamented  pro- 
ducts, the  common  occurrence  of  the  raw  material  of  this 
industry  has  led  potters  to  abandon  it  to  the  meanest 
uses,  although  the  decorative  possibilities  of  red-ware 
are  great. 

This  may  be  seen  in  the  Pennsylvania  Dutch  pottery 
of  the  last  century  and  in  the  red  faience  of  the  Rook- 
wood  pottery  of  to-day. 

The  former  ware,1  fashioned  of  red  clay,  was  dipped 
in  a  "slip"  of  white-burning  clay,  and  through  the  thin 
white  coat  thus  deposited  on  the  body  of  the  pieces,  de- 
signs were  etched  with  a  sharp  point,  appearing  as  dark 
red  lines  in  a  white  field,  resembling  the  sgraffito  mural 
decoration  of  Italy. 

The  ware  was  often  painted  with  mineral  colors ;  be- 
fore burning  it  was  covered  with  a  mixture  producing  a 
clear  glaze  in  the  fire. 

The  red  Rookwood  faience  is  made  of  a  highly  ferrugi- 
nous shale,  from  the  Ohio  river,  mixed  with  some  kao- 
lin, making  it  more  resisting  to  the  fire  and  giving  the 
baked  clay  a  lighter  and  more  brilliant  red  color.  It 
differs  from  ordinary  red  ware,  in  process  of  manufac- 
ture, in  that  the  clays  are  mixed  and  washed  and  the 
pieces  finished  in  two  fires,  as  in  the  case  of  yellow  ware, 
to  be  described  in  the  next  chapter. 

The  decoration  consists  in  clouded  grounds  of  under- 
glaze  colors,  applied  with  the  air-brush  and  hand-painted 
designs,  done  in  "slips"  stained  with  the  same  hard -fire 
colors. 


.  A.  Barber,  The  Pottery  and  Porcelain  of  the  United  States,  p.  66,  1893. 


RED   WARE.  63 

All  decoration  is  applied  to  the  freshly  formed  ware, 
when  in  a  leather-hard  condition,  and  the  dried  pieces 
are  baked  at  a  temperature  of  about  the  melting  down  of 
pyrometric  cone  seven. 

Drawn  from  this  kiln,  the  ware  is,  without  further 
treatment,  dipped  in  a  ferruginous  glaze,  having  the 
chemical  formula  : 


The  second  or  "glost"  fire  to  which  it  is  then  sub- 
jected being  about  of  the  temperature  necessary  to  melt 
silver. 

When  the  ground  color  consists  of  chrome  oxid  or 
iron  chrotnate  or  colors  containing  these,  a  prolonged 
or  repeated  glost  fire  may  convert  the  above  glaze  over 
them  into  an  aventurine  glass.  Being  nearly  saturated 
with  iron  it  will,  under  favorable  conditions  over  chrome- 
colors,  become  supersaturated  with  iron  chromate,  which, 
under  suitable  conditions  of  cooling,  separates  from 
the  glaze  in  crystalline  spangles,  which  have  a  brilliant 
golden  reflection. 

When  the  faces  of  the  crystals  are  turned  in  all  direc- 
tions the  glaze  often  resembles  aventurine  quartz  ;  when 
the  glaze  still  flowed  or  dragged  down  on  the  piece 
during  the  crystallization,  the  crystals  are  often  ranged 
in  rows  with  the  faces  turned  in  one  direction,  giving  a 
reflection,  as  do  the  filaments  of  asbestos  in  cat's-eye 
chalcedony. 


64  THE   CHEMISTRY   OF   POTTERY. 

Similar  crystallizations  can  be  produced  in  other  glazes 
and  on  other  bodies,  but  only  on  the  red  or  on  a  darker 
ground  and  through  the  medium  of  an  amber-yellow 
glass,  is  the  effect  striking  and  beautiful. 

EXAMINATION  OF  MATERIALS.—  If  the  rational  anal- 
ysis of  a  clay  and  its  general  appearance  indicate  that  it 
may  be  serviceable  for  making  red-ware,  tests  should  be 
made  to  determine  its  plasticity  and  its  binding  property. 

It  is  also  important  to  wash  a  larger  dried  sample 
through  a  set  of  graded  sieves  and  record  the  percenta- 
ges of  sand  and  other  coarser  impurities  remaining  on 
each,  in  order  that  the  potter  may  know  whether  these 
are  present  in  amounts  and  of  a  character  that  would 
present  mechanical  objections  in  fashioning  and  burning 
the  ware. 

Cakes  several  inches  square  and  about  T\  inch  thick, 
should  be  formed  and  dried  and  placed  in  a  muffle  with 
a  wire  of  pure  silver ;  on  one  or  two  of  the  cakes  such  a 
a  glaze-mixture  as  described  before  should  be  painted  to 
the  depth  of  about  -fa  inch.  The  mufflle  is  then  fired 
with  a  gradually  increasing  heat  until  in  the  course  of 
three  hours,  the  silver  wire  melts,  when  firing  is  discon- 
tinued. 

It  goes  without  saying  that  an  oxidizing  fire  must  be 
maintained  throughout  the  burning,  in  order  to  insure  a 
bright  red  color  of  the  clay  by  complete  oxidation  of  its 
iron  and  to  prevent  a  reduction  of  the  lead  of  the  glaze. 

When  cold  the  pieces  are  withdrawn  and  the  color  and 
hardness  of  the  unglazed  ( ' '  biscuit' ' )  pieces  and  the  char- 
acter of  the  glazed  sherds,  and  whether,  in  the  course  of 


RED  WARE.  65 

some  weeks  the  latter  bear  the  glaze  without  defect,  are 
noted. 

' '  Slips' '  for  ornamenting  wares,  as  described,  are  made 
by  softening  white  plastic  clays  or  mixtures  resembling 
the  white- ware  bodies  to  be  described  later  in  water  and 
passing  the  creamy  mixture  through  a  fine  sieve. 

For  engobing  ware,  as  in  the  case  of  the  Pennsylvania 
Dutch  potteries,  this  ''slip"  is  left  liquid  enough  to  allow 
the  pieces  to  be  immersed  in  it,  and  remain,  after  their 
withdrawal,  covered  with  a  smooth,  entirely  opaque  de- 
posit. 

For  painting,  the  slip  is  thickened  to  the  consistence  of 
common  tube-paints  and  colored  with  metallic  oxides. 

In  either  case  the  question  for  the  chemist  is  the  selec- 
tion of  a  white  clay  or  mixture  that  does  not  crack  or 
shell  off  from  the  particular  clay  upon  which  it  is  to  be 
applied,  during  the  processes  of  drying  and  firing. 

This  selection  can  only  be  made  by  empiric  trials,  as 
differences  in  shrinkage  both  during  the  first  step  of  dry- 
ing and  the  second  of  burning  between  the  body  and  the 
slip  applied  to  it,  differences  too  subtle  for  direct  meas- 
urement, would  be  fatal  to  the  result. 

The  trials  need  only  be  made,  however,  with  the  one 
object  of  uniformity  in  shrinkage  between  the  body  and 
the  slip.  The  latter  has  no  influence  on  the  crazing  or 
shivering  of  the  glaze,  which  is  applied  over  it,  provided 
that  the  body  is  suited  in  its  coefficient  of  expansion  to 
the  glaze. 


CHAPTER  VII. 
ROCKINGHAM  AND  YELLOW  WARE. 


HIGHER  grade  of  ware,  than  that  last  treated 
is  ' '  Rockingham  and  Yellow  Ware ; ' ' 
though  belonging  to  the  same  ceramic  cate- 
gory, namely,  "Faience,"  in  that  it  also 
consists  of  a  porous  body  covered  with  a 
transparent  lead  glaze. 
It  differs  from  ' '  red  ware ' '  in  color,  and  in  having  a 
body  requiring  a  higher  temperature  for  its  proper  burn- 
ing, so  that  the  glaze^s  not  applied  to  the  freshly  formed 
pieces  in  their  "clay  state,"  but  to  the  once  baked  or 
"biscuit"  pieces,  and  is  then  finished  in  a  second  or 
"glost"  fire  softer  than  the  first. 

The  clays  used  for  '  *  yellow  ware  ' '  belong  to  the  class 
commercially  known  as  second-class  fire  clays  ;  the  same 
from  which  common  firebrick  and  such  terra-cotta  arti- 
cles as  stove  and  flue-linings,  chimney-tops,  garden  vases, 
etc. ,  are  made.  They  are  generally  the  common  ' '  buff' ' 
or  "blue"  clays  of  the  coal  measures  and  are  widely 
distributed  in  all  our  carboniferous  exposures. 

A  typical  clay  of  this  character  has  the  following  com- 
position : 

ToTAiv  ANALYSIS. 

Insol.  in  H2SO4 
Per  cent.  Per  cent. 

Silica 60.50  22.33 

Alumina 25.53  0.53 


ROCKINGHAM    AND   YELLOW   WARE.  67 

Insol.  in  H2SO4. 
Per  cent.  Per  cent. 

Ferric  oxid 1.66  0.26 

Titanic  oxid 0.54  

Manganous  oxid 0.33  .... 

Lime 0.38  0.26 

Magnesia 1.19  0.07 

Alkalies 1 .76  0.37 

Combined  water 7.98  


99.87  23.82 

RATIONAL  ANALYSIS. 

Per  cent. 

Clay  substance 76.05 

Quartz 19.54 

Feldspathic  detritus 4.28 


^  99-87 

CHEMICAL  COMPOSITION  OF  THE  "  CLAY  SUBSTANCE." 

Per  cent. 

Silica 50. 19 

Alumina 32.87 

Ferric  oxid 1 .84 

Titanic  oxid ±    0.71 

Manganous  oxid 0.43 

Lime o.  16 

Magnesia 1.47 

Alkalies 1.82 

Combined  water ' 10.50 

The  yellow- ware  potter  is  better  equipped  in  machinery 
than  the  red- ware  potter,  and  does  not,  like  the  latter,  pre- 
pare his  clay  by  merely  soaking  it  with  water  and  then 
tread  and  knead  it  to  a  plastic  mass  of  the  proper  consist- 
ence ;  but  subjects  the  clay  to  a  regular  washing  process. 
Hence  the  presence  of  a  certain  amount  of  coarse  sand, 


68  THE   CHEMISTRY   OF   POTTERY. 

and  nodules  and  particles  of  iron  pyrites,  very  commonly 
found  in  all  such  clays,  do  not  spoil  them  for  his  work, 
as  they  are  removed  by  the  necessary  and  customary 
process  of  manufacture. 

This  consists  in  '  '  slipping  '  '  the  clay  in  a  vat  with  me- 
chanical stirrers,  known  as  a  "blunger,"  sifting  the 
"  slip"  through  a  sixty-mesh  wire  sieve  or  a  No.  8  silk 
lawn  stretched  over  a  vibrating  frame,  from  which  the 
coarser  sandy  impurities  are  thrown,  and  condensing  the 
/*  slip  "  to  plastic  clay  by  evaporation  or  by  a  filter-press. 

From  this  process,  it  will  be  realized  that  the  original 
clay  must  be  in  such  a  condition  of  physical  aggregation 
as  to  be  easily  disintegrated  or  '  '  slipped  '  '  by  mere  stir- 
ring in  water.  A  hard  clay,  readily  reduced  by  the  ele- 
ments in  "  weathering  "  to  such  a  condition,  will  also  an- 
swer the  purpose  ;  but  the  double  shoveling  involved  in 
transferring  clay  from  the  bank  to  a  weathering  flat,  and 
from  thence,  after  six  months  or  a  year's  exposure  to 
the  elements,  to  the  factory,  is  a  trouble  the  potter  avoids 
if  he  can. 

There  is  a  class  of  clays,  which  from  their  composi- 
tion are  admirably  adapted  for  yellow  ware,  but  have 
remained  completely  barred  from  this  use  because  of  their 
physical  character.  These  are  the  '  '  flint  clays'  '  of  which 
the  following  analysis  is  typical  : 

TOTAL  ANALYSIS. 

Insol.  in  HaSO4. 
Per  cent.  Per  cent. 


Silica  ..........................   55-°4 

Alumina  .......................  29.85  o.n 

Ferric  oxid  ....................      1.76  ---- 


ROCKINGHAM    AND   YELLOW   WARE.  69 

Insol.  in  H2SO4. 
Per  cent.  Per  cent. 

Lime 0.79  0.12 

Magnesia 0.57  0.17 

Alkalies 1.83  0.58 

Combined  water IO-95 


100.79  17-94 

RATIONAL  ANALYSIS. 

Per  cent. 

Clay  substance 82.85 

Quartz 16.58 

Feldspathic  detritus 1.35 


100.78 
PERCENTAGE  COMPOSITION  OF  THE  "CLAY  SUBSTANCE." 

Per  cent. 

Silica 45.97 

Alumina 35-9° 

Ferric  oxid 2.12 

Lime 0.81 

Magnesia 0.48 

Alkalies 1.51 

Combined  water '  13.22 

IOO.OO 

This  clay  occurs  in  rocky  masses  having  a  conchoidal 
fracture,  the  splinters  of  which  are  so  hard  and  sharp, 
that  a  flying  piece,  struck  off  with  the  pick,  will  cut  the 
hand  or  face.  It  eagerly  absorbs  water  and  with  a  crack- 
ling noise,  though  without  noticeable  evolution  of  heat, 
falls  to  pieces,  yielding  a  mass  of  shell-like  splinters, 
though  a  year's  exposure  to  the  weather,  while  reducing 
it  to  a  fine  sand,  fails  to  produce  a  workable  clay.  Simple 
grinding  in  water  and  thickening  the  resulting  "slip," 


70  THE    CHEMISTRY   OF    POTTERY. 

by  any  convenient  method,  produces  a  highly  plastic 
mass,  that  is  readily  formed  into  ware,  burns  to  a  bright 
yellow  color,  and  bears  the  customary  yellow  ware  glazes 
with  less  danger  of  ' '  crazing' '  than  the  before-mentioned 
and  commonly  used  clay.  This  latter  fact  is  to  be  ac- 
counted for  by  the  extreme  fineness  of  the  contained 
quartz,  more  than  compensating  for  the  reduced  quan- 
tity of  the  same. 

The  expense  of  wet-grinding  a  "flint  clay  "would  not 
be  excessive,  but  the  introduction  of  machinery  for  the 
purpose,  in  so  conservative  a  craft  as  the  one  under  con- 
sideration, would  be  met  with  almost  stolid  resistance. 

The  first  or  "biscuit  "  fire  of  yellow  ware,  for  harden- 
ing the  clay  pieces,  must  reach  "good  biscuit  heat," 
that  is  a  temperature  sufficient  to  bake  the  clay  so  hard 
that  it  can  no  longer  be  cut  with  a  knife,  but  that  the 
steel  leaves  a  lead-pencil-like  mark  on  the  surface.  At 
the  same  time,  the  sherd  must  still  be  porous  and  adhere 
when  touched  to  the  tongue. 

The  temperature  of  the  "  biscuit "  fire  will  necessarily, 
of  course,  vary  with  the  chemical  and  physical  character 
of  the  clay  used.  The  potter  determines  it  empirically 
by  testing -trials  of  the  clay  drawn  from  the  kiln,  with  his 
knife  and  tongue.  ' 

It  is  very  important  that  the  chemist  determine  at  what 
heat  the  clay  he  is  examining  attain  a  "good  biscuit" 
condition,  in  order  that  the  potter  may  know  whether  he 
can  introduce  ware  made  of  it  into  his  biscuit  kiln  along 
with  his  old  ware.  As  a  kiln  of  ware  represents  consid- 
erable capital  exposed  to  a  risky  operation,  potters  nat- 


ROCKINGHAM    AND   YKLLOW   WARE.  7! 

urally  are  absolutely  opposed  to  the  adoption  of  new 
clays  requiring  different  conditions  of  fire  from  those 
already  adopted  by  them  ;  these  must  dovetail  easily  at 
least  into  their  kiln  conditions,  by  requiring  the  same 
heat. 

Yellow-ware  clays  reach  the  required  hardness  at  tem- 
peratures varying  from  the  melting  points  of  pyrometric 
cones  five  to  seven  or  even  eight. 

For  ' '  yellow  ware  "  as  for  "  red  ware, "  it  is  customary 
to  employ  a  "  raw  "  glaze,  that  is,  one  containing  no  sol- 
uble constituents  which  have  first  to  be  rendered  insolu- 
ble by  fritting  to  a  glass. 

The  customary  type  of  glaze  has  a  formula  resembling 
one  of  the  following:  i  PbO,  0.2  A12O3,  2  SiO2,  which 
may  be  made  by  grinding  together  129.7  white  lead, 
25.9  china  clay,  and  48  flint. 


o.SPbO 
o.2K20 


|o.2A!2O3,  2SiOa  from 


i.o 

103.8  white  lead,  55.7  feldspar,  and  24  quartz. 
o.SPbO  } 

o.iCaO    [o.2A!2O3,  2SiO2. 
o.iK2O  ) 

i.o 

obtained  by  grinding  together  103.8  white  lead,  27.9 
feldspar,  13  china  clay,  5  calcium  carbonate,  and  36 
quartz. 

The  acidity  of  the  glaze  generally  varies  from  one  and 


72  THE    CHEMISTRY   OF   POTTERY. 

eight-tenths  to  two  and  two-tenths  SiO2,  depending  upon 
the  amount  of  quartz  contained  in  the  clay  upon  which 
it  is  expected  to  stand.  The  alumina  will  similarly  vary 
from  o.i 6  to  0.2  and  over  according  to  the  stiffness  of 
the  glaze  required  and  the  conditions  of  firing,  causing 
a  liability  to  devitrification,  or  most  likely  as  chance 
has  thrown  a  fairly  satisfactory  formula  into  the  potter's 
hands. 

The  temperature  at  which  the  glazes  run  bright  lies 
about  at  a  point  at  which  an  alloy  composed  of  fifty  per 
cent,  silver  and  fifty  per  cent,  gold  will  melt,  though 
depending  upon  the  composition  of  the  glaze,  and  the 
length  of  fire,  the  melting  point  of  the  alloy,  seventy-five 
per  cent,  gold  and  twenty-five  per  cent,  silver  may  have 
to  be  reached. 

' '  Rockingham  ware ' '  differs  from  ' '  yellow  ware' '  only 
in  that  it  is  covered  with  a  brown  manganiferous  glaze, 
applied  either  by  spattering  the  piece,  previously  dipped 
in  the  clear  glaze,  with  the  same,  thus  producing  a  mot- 
tled effect  by  the  melting  of  the  glazes  into  each  other, 
or  by  directly  dipping  the  biscuit  piece  into  the  ' '  Rock- 
ingham" glaze  alone,  whereby  the  fired  piece  obtains  a 
uniform  red-brown  finish. 

Common  forms  of  Rockingham  glazes  are  the  following  : 


,I5MnO    '  -.-..<-«SiO, 


i.oo 

made  by  grinding  together    no  white  lead,  6.5   man- 
ganese dioxid,  23.3  china  clay,  and  43.2  flint; 


ROCKINGHAM    AND   YEUvOW   WARE.  73 

also, 

o.9PbO   )o.i5A!203 
o.iMnO  jo.o5Fe203 

i.o 

similarly  prepared  with  a  ferruginous  clay,  the  formula 
of  which  any  chemist  will  be  able  to  figure  out  from  the 
equivalent  proportions  given. 

As  in  the  case  of  ' '  red  ware ' '  the  prevailing  fire  of 
both  the  biscuit  and  glost  kilns  should  be  oxidizing. 

PRODUCTS. — The  ware  manufactured  in  this  industry 
embraces  such  kitchen  and  other  domestic  utensils  as 
bowls,  bakers,  nappies,  chambers,  tea  and  coffee-pots, 
pitchers,  etc. 

Classes  of  ornamental  pottery  closely  allied  to  yellow- 
ware  and  readily  developed  in  connection  with  it,  inas- 
much as  they  require  similar  raw  materials  and  condi- 
tions of  firing,  are  those  known  as  * '  Limoges' '  and  ' '  Bar- 
botine." 

In  both  of  these,  the  freshly  formed  ware  is  decorated 
by  painting  with  "slips"  of  white  plastic  clay,  mixed 
with  under-glaze  colors.  In  Barbotine  ware,  this  deco- 
ration is  supplemented  with  ornaments  and  flowers  mod- 
eled in  high  relief  directly  on  the  pieces. 

Thus  finished,  the  ware  is  dried  and  burned  in  biscuit, 
after  which  treatment,  it  is  covered  with  a  clear  yellow- 
ware  glaze  and  burned  in  the  glost-kiln. 

EXAMINATION  OF  THE  MATERIALS. — In  examining 
a  clay  for  its  possible  use  in  this  manufacture  it  is  im- 
portant to  determine  whether  it  can  readily  be  disinte- 


74  THE   CHEMISTRY   OF   POTTERY. 

grated  by  stirring  or  boiling  in  water,  or  whether  repeated 
soaking  and  drying  or  freezing  and  thawing  bring  it  to 
such  a  state. 

A  ' '  slip ' '  of  the  clay  should  be  run  through  a  sixty 
mesh  wire  sieve  and  the  amount  and  character  of  the  re- 
maining sand  given. 

If  the  latter  is  at  all  appreciable  the  sample  for  analysis 
should  not  be  taken  from  the  crude  clay,  as  this  would, 
of  course,  give  an  entirely  false  idea  of  the  constituents 
ultimately  entering  the  potter's  "  body,"  but  the  sample 
should  be  taken  from  the  thickened  and  dried  "slip" 
which  has  passed  the  sixty  mesh  sieve.  Such  a  "slip" 
is  readily  thickened  by  pouring  it  on  a  thick,  clean,  and 
dry  slab  of  plaster  of  Paris,  which  readily  absorbs  the  water 
and  from  which  the  plastic  clay  is  easily  peeled  without 
danger  of  contamination  with  plaster. 

A  part  of  the  now  plastic  clay  is  dried  for  preparation 
of  the  sample  for  a  rational  analysis,  in  the  customary 
manner.  The  bulk  of  the  washed  product  is  formed  into 
cakes  and  rings  for  the  burning.  The  latter  are  made 
for  the  purpose  of  watching  the  progress  made  by  the 
clay  in  the  fire,  and  determining  when  the  baking  is  fin- 
ished. These  rings  should  be  of  such  size,  as  to  be  readily 
withdrawn  from  the  muffle  with  a  stout  iron  wire,  through 
the  spy-hole  in  the  door  of  the  same. 

When  the  clay  pieces  are  bone-dry,  they  are  placed  in 
the  muffle  on  a  thick  bed  of  clean  quartz  sand  or  a  thick 
fire-clay  tile  strewn  with  the  same. 

The  rings  should  be  so  placed  as  to  be  easily  reached 
through  the  spy-hole  in  the  door  brick  and  parallel  with 


ROCKINGHAM    AND    YELLOW    WARE.  75 

them  the  Seger  cones  five,  six,  seven  and  eight,  should 
be  set  upright  on  a  piece  of  tiling  or  flat  cake  of  baked 
or  raw  clay,  to  which  they  had  best  be  stuck  with  a  little 
"slip."  The  door  brick  is  then  luted  in  place  with  a 
wad  of  soft  clay,  and  the  firing  begun  gradually,  raising 
the  heat  at  an  increasing  rate. 

Should  there  be  difficulty  in  seeing  the  pyroscopes  when 
the  muffle  has  reached  bright  redness,  on  removing  the 
plug  from  the  spy-hole,  the  careful  introduction  of  a  thick 
iron  wire  in  their  neighborhood,  will  momentarily  so  far 
reduce  their  temperature  as  to  make  them  clearly  discern- 
ible against  the  bright  walls  of  the  muffle. 

When  cone  five  crooks  over,  the  first  clay  ring  should  be 
drawn  from  the  muffle  with  a  wire%nd  when  cool,  tested 
with  the  knife  for  hardness;  if  insufficient  the  firing  is 
continued  until  cone  five  has  melted  down  completely, 
when  the  second  ring  is  drawn.  The  next  trial  is  drawn 
at  the  crooking  of  cone  six,  and  so  on  until  a  heat  is  reached 
at  which  the  clay  trial  ring  is  sufficiently  hard  to  resist 
cutting  with  a  knife,  without  having  lost  all  porosity, 
adhering  when  touched  to  the  tongue. 

When  this  heat  is  reached  the  firing  is  discontinued 
and  the  furnace  allowed  to  cool. 

The  fired  pieces  of  clay  removed  from  the  cold  muffle 
are  covered  on  one  side  with  one  of  the  described  glazes, 
of  which  the  first  is  recommended,  being  easily  made 
and  fired  as  a  convenient  empirical  standard,  against 
which  to  test  the  coefficient  of  expansion  of  the  baked 
clay. 

The  pieces  are  then  baked  a  second  time  to  melt  the 


76  THE   CHEMISTRY   OF   POTTERY. 

glaze  upon  them,  the  heat  reached  being  that  of  the 
melting-point  of  the  alloy,  fifty  per  cent,  silver  and  fifty 
per  cent.  gold. 

It  is  also  very  convenient  to  use  the  small  baked  trial 
rings  partly  covered  with  the  glaze  as  trials  for  this  fire, 
the  firing  being  discontinued  when  the  glaze  on  one  of 
these  rings  drawn  from  the  muffle  has  run  bright. 

The  color  of  the  clay  under  the  clear  glaze  should  be 
described.  Tints  approaching  bright  straw  and  lemon- 
yellows  are  the  ones  sought  after.  Brownish  and  reddish 
tints  are  not  acceptable  to  the  trade  buying  the  ware. 

The  behavior  of  the  glaze  on  the  clay  must  also  be 
given.  From  these  data  the  potter  will  be  able  to  deter- 
mine whether  the  acidity  of  his  glaze  must  be  increased 
or  diminished,  or  which  clays  to  mix,  if  he  choose  to 
keep  the  glaze  as  it  is  and  compensate  the  incorrect  co- 
efficient of  expansion  of  the  clays  by  mixing  them. 

Slips  for  underglaze  painting  in  the  Limoges  man- 
ner must  be  found  empirically  for  the  particular  yellow- 
ware  body  upon  which  they  are  to  be  used. 

The  clays  for  executing  the  high-relief  Barbotine 
modeling  must  bear  the  glaze  that  is  finally  to  be  applied 
to  the  ware  without  fault.  They  are  usually  mixed 
after  the  manner  of  white- ware  bodies,  to  be  described 
in  a  subsequent  chapter.  A  number  of  such  mixtures 
must  be  found  that  will  bear  the  yellow- ware  glaze,  and 
among  these  such  a  one  is  selected  that  will  in  its  shrink- 
ages conform  with  those  of  the  yellow  body  upon  which 
it  is  to  be  applied. 


CHAPTER  VIII. 
STONEWARE. 

HE  products  belonging  to  Brogniart's  second 
class,  sixth  division,  are  perhaps  next  to 
common  building  brick,  the  most  important 
of  ceramic  manufactures. 

Such  articles  as  milk  and  butter  crocks, 
jars  for  acid  liquids,  sewer-piping,  and  vit- 
reous paving  brick  being  needed  in  large 
quantities  and  the  purposes  to  which  they  are  put,  pre- 
cluding other  than  the  very  cheapest  wares,  the  sole  aim 
of  American  manufacturers  has  turned  in  the  direction 
of  improvement  in  the  mechanical  engineering  of  their 
plants  and  not  in  that  of  materially  improving  the  ware 
itself. 

But  even  in  the  engineering  of  such  factories  there  is 
still  great  room  for  progress.  Thus  it  is  unlikely  that 
there  is  as  yet  a  single  continuous  kiln  in  operation  in 
one  of  the  branches  of  this  industry  in  the  country, 
although  the  relative  cost  of  fuel  to  product  is  such  that 
the  heat  wasted  in  intermittent  kilns  represents  an  ap- 
preciable percentage  of  cost. 

And  further,  as  the  ware  is  piled  up  openly  in  the 
kiln,  without  the  protection  of  saggars  and  is  frequently 
damaged  by  the  ferruginous  ash,  carried  into  the  kilns 
from  the  firings  by  the  strong  draught,  the  fuel  should 


78  THE   CHEMISTRY   OF   POTTERY. 

be  converted  into  gas  in  producers,  before  being  ad- 
mitted to  the  kilns.  This  can  only  be  rationally  done  in 
conjunction  with  continuous  firing. 

Besides  cheapening  the  production,  chemists  must  be 
particularly  interested  in  improving  the  quality  of  stone- 
ware, as  it  is  the  material  in  which,  on  a  manufacturing 
scale,  all  those  operations  are  performed  which  in  the 
laboratory  require  the  use  of  porcelain,  glass,  and  plati- 
num. 

Our  chemical  industries  cannot  attain  great  propor- 
tions until  stoneware  condensing  jars  and  worms,  tanks, 
siphons,  plate-towers,  acid-pumps,  pyrites  roasting 
plates,  are  as  plentifully  and  cheaply  manufactured  here 
and  of  as  good  quality  as  abroad. 

As  wrought  iron  has  artistic  qualities  that  in  its  way 
cannot  be  equaled  by  more  costly  metals,  so  stoneware 
has  decorative  possibilities  far  beyond  those  of  the  various 
white  wares,  in  faience  or  even  porcelain,  upon  which 
too  much  ingenuity  has  been  spent.  The  old  German 
and  Flemish  stonewares  amply  prove  this,  as  do  also, 
in  their  several  ways,  the  modern  productions  of  Doul- 
ton  in  Lambeth,  Villeroy  and  Boch  in  Mettlach  and  Mer- 
zig  and  the  Banko-ware  of  Japan. 

In  the  main,  the  same  class  of  clays  as  those  used  in 
the  yellow- ware  industry,  are  employed  for  making 
stoneware.  The  difference  in  the  body  of  the  two  prod- 
ucts being  the  result  of  the  difference  in  temperature  to 
which  they  are  subjected. 

The  body  of  yellow  ware  is  burned  until  it  can  no 
longer  be  scratched  with  a  steel  poiift,  but  it  must  still 


STONEWARE.  79 

be  sufficiently  porous  to  adhere  strongly  when  touched 
to  the  tongue,  and  it  must  not  have  lost  its  bright  yel- 
low color.  Stoneware  is  burned  until  the  clay  has  be- 
come vitreous  and  is  of  a  greyish  stone  color. 

Clay  suitable  for  stoneware  must  be  free  from  stony 
concretions  and  larger  particles  of  iron  pyrites,  as  it  is 
not  customary,  in  this  work,  to  purify  the  clay  as  the 
yellow- ware  potter  does. 

In  the  country  shops  the  potter  merely  soaks  and 
tramps  his  clay  to  homogeneity  ;  in  the  factories  the 
same  is  accomplished  by  grinding  the  wet  clay  under 
runners  in  the  horizontal  ' '  wet-pan . ' ' 

In  many  of  the  newer  factories,  it  is  true,  the  clay 
which  is  to  be  formed  in  plaster  molds  is  subjected  to  a 
washing  process,  as  in  the  preparation  of  the  material 
of  the  industry  last  described.  But  those  who  form 
their  ware  free-hand  on  the  potter's  wheel  insist  that  a 
clay  loses  much  of  its  plasticity  in  washing,  becoming 
"punkjV  that  is,  elastic  rather  than  plastic.  Whether 
this  is  merely  a  conservative  prejudice  or  has  an  element 
of  truth  in  it,  has  not,  as  yet,  been  proven. 

The  requirement  remains  that  a  clay  for  stoneware 
must  be  pure  enough  not  to  need  washing. 

The  first  clay  of  which  the  analysis  is  given,  in  the 
preceding  chapter,  is  suitable  for  stoneware  and  is  used 
for  the  purpose.  However,  a  clay  a  little  richer  in 
alkali  and  rather  more  siliceous  would  be  better,  espec- 
ially if  the  ware  is  to  be  salt-glazed,  as  it  would  then 
come  of  a  more  uniform  stone-grey  color  and  take  the 
salt-glazing  more  perfectly. 


80  THE   CHEMISTRY   OF   POTTERY. 

The  following  analysis  is  of  the  clay  used  in  one  of 
the  larger  factories,  making  a  ware  of  typical  character : 

The  entire  clay  Portion  insoluble  in 

contains  H2SO4  contains 

Per  cent.  Per  cent. 

Silica 71.58  46.06 

Alumina 18.31  0.43 

Ferric  oxid 1 .09  0.04 

Lime 0.40  0.23 

Magnesia 0.62  o.io 

Alkalies1 2.96  0.63 

Combined  water 5.95  o.oo 

Manganous  oxid trace  o.oo 


100.91  47.49 

RATIONAL,  ANALYSIS. 

Per  cent. 

Clay  substance 53«42 

Quartz 44.41 

Feldspathic  detritus 3.08 


100.91 
PERCENTAGE  COMPOSITION  OF  THE  CI,AY  SUBSTANCE. 

Per  cent. 

Silica 47.70 

Alumina 33.47 

Ferric  oxid 1.96 

Ivime 0.31 

Magnesia 0.97 

Alkalies 4.36 

Combined  water 11.13 

99.90 

The  alkali  in  the  above  clay  is  about  as  high  as  it  dare 
be   under  ordinary   circumstances,  for  as  stoneware  is 


1  Combining  weigfft  of  the  alkalies  47.00  per  cent. 


STONEWARE.  8 1 

burned  to  vitreousness,  there  is  danger  of  the  pieces  col- 
lapsing in  the  fire,  if  the  clay  be  too  fusible. 

Like  all  vitreous  ware,  stoneware,  unless  it  be  quite 
thin  and  very  evenly  formed  as  are  porcelain  crucibles, 
dishes,  and  beakers  for  laboratory  use,  is  sensitive  to  sud- 
den heating  and  cooling. 

Cooking-crocks  and  such  other  vessels,  which  are  to  be 
subjected  to  these  conditions,  are  made  to  come  hard,  but 
still  quite  porous  in  the  stoneware  fire.  They  are  not 
stoneware,  properly  speaking,  though  generally  manu- 
factured with  it. 

The  clays  used  for  making  such  products  contain  a 
large  percentage  of  sharp  quartz  sand.  WheVe  they  do 
not  occur,  they  may  be  made  by  mixing  quartz  sand  with 
a  clay  that  is,  in  the  main,  more  aluminous  and  less  rich 
in  alkali  than  a  stoneware  clay. 

Stoneware  is  fired  but  once,  its  glaze  being  applied 
before  or  during  that  operation,  and  melted  while  the  clay 
is  hardening  to  the  proper  character.  It  is  either  salt-  or 
slip-glazed — that  is,  it  has  either  a  very  thin  blush  of  a 
soda-glass  on  its  surface,  produced  by  volatilizing  com- 
mon salt  in  the  kiln,  when  the  fire  has  reached  its  high- 
est point,  or  before  placing  in  the  kiln,  it  is  coated  with 
a  fusible  ferruginous  clay  which  melts  -at  the  heat  in 
which  the  body  of  the  ware  is  properly  baked.  Most 
stoneware  is  salt-glazed  on  the  outside  and  slip-glazed 
on  the  inside  where  the  salt- vapor  can  not  reach,  owing 
to  the  fact  that  the  pieces  set  on  top  of  one  another  in 
the  kiln  mutually  protect  their  inner  surfaces  from  the 
kiln  atmosphere. 


82  THE   CHEMISTRY   OF   POTTERY. 

The  stoneware  potter  fires  his  kilns  altogether  by 
' '  slip-trials ; ' '  these  are  pieces  of  his  clay  dipped  in  the 
"slip"  or  glazing-clay  and  suitably  placed  in  the  kiln 
opposite  the  trial  holes,  where  they  can  be  reached  with 
an  iron  hook  and  withdrawn  one  by  one  for  inspection 
as  the  fire  progresses.  When  the  slip- clay  has  thor- 
oughly melted  and  attained  a  bright  brownish-black 
color,  the  proper  heat  is  considered  to  be  reached. 

If  the  ware  is  to  be  salt -glazed,  a  certain  amount  of 
salt  is  then  thrown  at  regular  intervals  into  the  kiln- 
mouths,  together  with  green  wood,  which  gives  the  water 
vapor  necessary  to  decompose  the  fumes  of  sodium  chlorid 
and  set  the  alkali  free  for  combination  with  the  alumina 
and  silica  on  the  surface  of  the  body  of  the  ware. 

If  the  ware  is  entirely  slip-glazed,  the  firing,  on  reach- 
ing the  proper  temperature,  is  merely  discontinued. 

In  almost  all  of  the  stoneware  potteries  of  the  United 
States,  the  temperature  reached  is  that  of  the  melting  of 
Seger's  pyrometric  cone  eight. 

The  reason  of  the  rather  surprising  uniformity  of  the 
heat  adopted  in  practically  all  of  the  establishments  mak- 
ing this  ware,  and  irrespective  of  the  clays  they  are  using 
for  the  body  of  the  product,  lies  in  the  fact  that  this  is 
the  heat  required  to  melt  one  particular  slip-clay.  This, 
on  account  of  its  convenience  in  several  of  its  physical 
properties,  particularly  on  account  of  its  great  uni- 
formity in  color  and  melting-point,  is  used  in  nearly  all 
of  the  stoneware  potteries  of  the  country  for  glazing  their 
ware.  Even  where  other  slip-clays  are  used  for  this  pur- 
pose, the  temperature  of  the  kilns  was  at  least  originally 


STONEWARE.  83 

adopted  for  the  use  of  this  slip,  and  it  is  used  for  the  firing 
trials,  if  not  for  glazing  the  ware  itself. 

An  analysis  of  this  clay,  which  is  mined  near  Albany, 
New  York,  and  is  commonly  known  as  "  Albany  slip," 
runs  as  follows : 

The  entire  clay    The  portion  insoluble 
contains  in  H2SO4  contains 

Per  cent.  Per  cent. 

Silica 58.54  38.36 

Alumina 15.41  2.33 

Ferric  oxid 3.19  0.12 

Lime 6.30  i  .42 

Magnesia 3.40  o.  15 

Alkalies1 4.45  2.65 

Carbon  dioxid 6.85  .... 

Sulfur  trioxid i .  10  .... 

Phosphorus  pentoxid trace  .... 

Combined  water 1.23  

100.47  45-03 

RATIONAL  ANALYSIS. 

Per  cent. 

Clay  substance 39-35 

Feldspathic  detritus i5-3>* 

Quartz 29.72 

Calcium  sulfate 1.87 

Calcium  carbonate 7.09 

Magnesium  carbonate 7.13 

100.47 
PERCENTAGE  COMPOSITION  OF  THE  CI,AY  SUBSTANCE. 

Per  cent. 

Silica 5LQ9 

Alumina 33.12 


i  Combining  weight  45.8. 


84  THE   CHEMISTRY   OF   POTTERY. 

Per  cent. 

Ferric  oxid 7-7° 

Lime 0.35 

Alkalies  4-55 

Combined  water • 3.11 


99.92 

As  this  clay  is  used  upon  the  ware  as  a  glaze,  it  is  im- 
portant to  calculate  from  its  gross  analysis  the  chemical 
formula  that  the  glass  resulting  from  melting  it  will  have, 
in  order  to  find  the  type  of  glaze  required  in  this  industry; 
to  be  able  to  reproduce  it  from  other  materials ;  or  to  sys- 
tematically modify  it  for  the  purpose  of  producing  glazes 
of  lower  or  higher  melting-points,  to  accommodate  clays 
that  should  be  burned  at  temperatures  other  than  those 
required  for  this  particular  slip-glaze. 

The  chemical  formula  of  the  Albany  slip-glaze,  calcu- 
lated from  the  above  analysis,  would  be : 


i.o     RO          0.689    R2O3 

The  following  slip-glaze,  which  is  also  burned  at  about 
the  same  heat  with  the  Albany,  becomes  of  a  rich  red- 
brown  color ;  it  is  composed  of  a  mixture  of  fusible  clays, 
the  proportions  having  been  established  empirically,  are 
kept  secret  and  are  unknown  to  the  writer. 

Per  cent. 

Silica     55-67 

Alumina 14.18 

Ferric  oxid •  •     3-5^ 

Lime   8.00 

Magnesia 2.84 


STONEWARE.  85 

Per  cent. 

Manganous  oxid 0.41 

Alkalies1 5.01 

Sulfur  trioxid i.n 

Combined  water  and  carbon  dioxid   9.87 


100.65 

The  chemical  formula  of  the  glaze  resulting  from  melt- 
ing the  above  would  be  : 
0.2290  KNaO  ^ 
0.0203  MnO      I  0.4832  A1203 
0.2492  MgO      \  0.078l  Fe2o 


i.o     RO  0.5613    R2O3 

It  appears  from  these  formulas  that  the  common  slip- 
glaze  for  stoneware  must  have  a  chemical  formula  not 
differing  widely  from  that  of  Seger's  pyrometric  cone 
number  two,  namely  : 


o.75CaO/o.iFe2O3 
i.o    RO     o.5R2O3 

As  clays  high  in  mineral  detritus  are  common,  abound- 
ing especially  along  the  borders  of  former  glaciation,  the 
chemist  may  frequently  be  called  upon  to  show  the  stone- 
ware potter  how  a  local  slip-clay  should  be  mixed  in 
order  to  melt  in  a  fire  most  suitable  for  the  body  of  his 
ware. 

It  is  commonly  believed  by  potters,  and  also  by  chem- 
ists, that  if  a  clay  be  infusible,  it  is  only  necessary  to  add 
sufficiently  of  some  basic  material,  as  an  alkali,  alkaline 


Combining  weight  38.4. 


86  THK   CHEMISTRY   OF   POTTKRY. 

earth,  litharge  or  ferric  oxid  to  obtain  a  glass.  This  is 
not  the  case,  as  the  relation  of  the  R2O3  elements  to  silica 
is  vital  and  a  glaze  of  this  type  may  be  infusible  from 
being  too  basic.  The  chemist  on  working  out  the  chem- 
ical formula  of  the  clay,  which  it  is  desired  to  use,  will 
frequently  find  it  to  be  too  aluminous,  requiring  the  ad- 
dition of  silica  quite  as  much  as  that  of  a  base. 

In  order  to  illustrate  this,  systematic  experiments  were 
made  with  a  slip-clay  that  merely  softened  and  swelled 
up  at  the  melting  down  of  pyrometric  cone  eight,  but  was 
far  too  infusible  to  run  at  that  heat.  It  analyzed  as  fol- 
lows : 

The  portion  in- 

The  entire  clay  soluble  in  H2SO4  and 
contains  Na2CO3  contains 

Per  cent.  Per  cent. 

Silica 56.59  23.56 

Alumina 25.96  i.oo 

Ferric  oxid 2.30  0.07 

Ljme    i.  60  0.06 

Magnesia 1.90  o.io 

Alkalies1 5.33  0.40 

Combined  water  ....       6.22                     •  ««« 
Sulfuric  anhydrid ...       i  .07  

100.97  25.19 

RATIONAL  ANALYSIS. 

Per  cent. 

Clay  substance 73-96 

Feldspathic  detritus 5-44 

Quartz 19-75 

Calcium  sulfate i  .82 


100.97 


1  Combining  weight  31. 


STONKWARE.  87 

PERCENTAGE  COMPOSITION  OF  THE  CI,AY  SUBSTANCE. 

Per  cent. 

Silica  .............................  44-65 

Alumina  ..........................  33-74 

Ferric  oxid  ........................  3.01 

Lime  ..............................  i  .09 

Magnesia  .........................  2.43 

Soda  ..............................  6.66 

Combined  water  .  .  £  ...............  8.40 

CHEMICAL  FORMULA  OF  THE  MEI/TED  CI<AY. 


i.o        RO        1.6437  R2O3 

Combining  weight  of  the  clay  according  to  this  for- 
mula, 308.5. 

In  order  to  make  a  slip-glaze  with  this  clay  the  fol- 
lowing trial  mixtures  were  ground  with  water  in  a  mor- 
tar and  poured  over  cakes  of  stoneware  clay,  in  a 
leather-hard  condition,  forming  a  deposit  on  each  one 
millimeter  thick.  The  pieces  were  subjected,  after  dry- 
ing, to  the  heat  of  the  stoneware  kiln. 

A.  0.1768  Nap  ]  ] 

?$  fcfsa 

CaO    j  J    j 


i.o       RO         o.6i8R2O3 
made  by  grinding  together 

102.8  parts  of  the  slip  clay. 
33-3      "      "          whiting. 
61.8      "      "          flint. 
5.6      "       "  ferric  oxid. 


88  THE   CHEMISTRY   OF   POTTERY. 

B. 

0.2826  KNaO    1  1 

0.6441  CaO        t°-5387  * 
0.0733 


i.o       RO  0.6409  R2O3 
made  of 

77.1  parts  of  th^slip  clay. 

41.5  "  "           feldspar. 

30.0  "  "          whiting. 

19.0  "  "          flint. 

6.4  "  "          ferric  oxid. 

C.  I/ike  B,  but  with  increased  silica. 

i.oRO     0.6409  R2O3     4SiO2. 

taking  for  the  mixture 

77.1  parts  of  the  slip  clay. 
41.5      "      "          feldspar. 
30.0      "      "          whiting. 

49.0  "      "          flint. 

6.4      "      "  ferric  oxid. 

D.  With  a  further  increase  of  silica  containing 

77.1  parts  of  the  slip  clay. 
4L5      "      "          feldspar. 
30.0      "      "          whiting. 
79.0      "      "          flint. 

6.4      "      "          ferric  oxid. 

A,  containing  insufficient  alkali,  melted  to  a  puckered 
greenish-brown  coat,  with  little  in  its  appearance  to 
characterize  it  as  a  glass.  The  increased  alkali  in  the 
following  trials  overcame  this  defect,  they  being  bright 
and  glass-like. 


STONEWARE.  89 

B,  while  presenting  the  appearance  of  a  dark-brown 
glass,  was  devitrified  at  the  edges  and  contained  through- 
out whitish  crystalline  separations  resembling  the  ' '  fig- 
ging" of  certain  transparent  soaps. 

D  was  insufficiently  fused,  but  appeared  otherwise  as 
a  perfect  glass. 

C  presented  a  brownish-black,  highly  lustrous  glass, 
having  in  every  particular  the  character  of  a  perfect 
stoneware  slip-glaze. 

These  experiments  show  that  any  fusible  clay  can  be 
mixed  to  make  a  "  slip-glaze."  The  use  of  a  particular 
clay  for  the  purpose  is  only  profitable,  however,  when  it 
is  quite  low  in  alumina  and  very  high  in  alkalies. 

Alkaline  earths,  ferric  oxid,  and  silica  are  easily  and 
cheaply  added  as  powdered  limestone,  iron  ore,  and 
sand,  frequently  found  of  sufficient  purity  where  such 
clays  occur.  Feldspar  is  the  only  practical  source 
of  alkali ;  if  the  clay  is  deficient  in  this  constitu- 
ent, bearing  alumina  as  well,  it  often  imparts  too  much  of 
the  latter  to  the  mixture,  when  added  in  sufficient 
amount  to  give  the  required  alkali,  unless  the  propor- 
tion of  the  clay  be  depressed  to  practical  insignificance. 

This  is  the  difficulty  with  the  slip-clay  used  in  the 
above  experiments.  While  the  alkali  is  in  itself  high, 
the  percentage  of  alumina  is  too  great,  making  the  for- 
mer small  in  proportion  to  it.  The  feldspar  necessary 
to  bring  about  a  proper  ratio  of  alkali  to  alumina  de- 
presses the  allowable  proportion  of  the  clay  to  less  than 
forty  per  cent,  of  the  mixture,  and  even  in  this  case  the 
R2O3  constituents  are  high,  as  they  should  rather  ap- 
proach 0.5  equivalent,  than  exceed  0.6. 


90  THE    CHEMISTRY   OF   POTTERY. 

There  is  a  belief  among  potters  that  the  glaze  pro- 
duced by  the  melting  of  a  slip-clay  will  not  ' '  craze  ' '  on 
any  body  ;  that  other  compounds  producing  glasses  are 
of  a  nature  antagonistic  to  clays,  while  slip-clays,  being 
clays  themselves,  have  an  affinity  for  them. 

This  idea  is,  of  course,  quite  absurd,  as  in  all  cases 
the  perfect  union  of  bodies  and  glazes  is  identity  of 
their  coefficients  of  expansion  within  the  limits  of  elas- 
ticity. It  is  true,  however,  that  the  alkali-alkaline- 
earth  glasses  allow  a  greater  range  of  composition  of  the 
body  between  the  manifestations  of  crazing  and  shiver- 
ing, than  do  lead-glasses  particularly. 

In  salt-glazing,  as  already  stated,  the  body  of  the  ware 
itself  furnishes  the  necessary  alumina  and  silica  for  the 
glaze.  The  clay  should,  therefore,  contain  these  ele- 
ments in  proportions  most  favorable  to  the  production  of 
a  hard-fire  glaze.  As  already  seen,  such  glazes  approx- 
imate a  formula 

iROo.5R2O34SiO2. 

The  yellow-ware  clay  mentioned  in  the  beginning  of 
the  chapter  as  used  for  stoneware,  although  it  does  not 
take  the  salt-glazing  well,  has  the  proportion  0.5  R2O3to 
2.034  SiO2.  It  is  therefore  too  aluminous  to  take  a  good 
salt-glaze. 

In  the  other  stoneware  clay  given,  the  proportion  is 
o.5R2O3  to  3.35  SiO2,  which  very  nearly  coincides  with 
what  is  the  best  proportion  of  these  constituents  in  the 
glazes  and  accounts  for  the  better  formation  of  the  salt- 
glaze  upon  it. 


STONEWARE.  91 

One  other  point,  however,  also  enters  into  the  consid- 
eration of  the  likelihood  of  a  clay's  accepting  the  salt- 
glaze  well,  and  that  is,  that  it  contain  sufficient  fluxing 
material  to  become  vitreous  at  a  reasonable  heat.  For 
if  the  clay  be  still  porous  when  the  alkali  fumes  are  intro- 
duced into  the  kiln,  these  will  be  absorbed  by  the  ware 
without  glazing  its  surface. 

Recently,  the  demand  for  a  neater  looking  interior 
glaze  for  stoneware,  than  the  dark-colored  slip-glaze  has 
led  to  the  introduction,  by  a  few  factories,  of  whitish 
semi-opaque  glazes.  These  are  of  two  types,  the  one 
being  a  lead  glass,  the  other  an  alkali — alkaline  earth 
glass,  but  both  contain  zinc  oxid  as  the  ingredient 
giving  the  distinctive  character.  One  of  these  analyzed 
as  follows : 

Per  cent. 

Zinc  oxid JS-S1 

Lime 7.47 

Magnesia  0.20 

Alkalies   1.56 

Insoluble  in  dilute  hydrochloric  acid  70.03 

Loss  on  glowing 8. 14 

100.71 
The  insoluble  portion : 

Per  cent. 

Alumina 13-83 

Ferric  oxid o.  14 

Lime    0.60 

Magnesia 0.14 

Potash 7.47 

Silica    47.85 

70.05 


92  THE   CHEMISTRY   OF    POTTERY. 

This  would  give  a  glass  with  the  chemical  formula : 

o.2335K20  ^ 

0.3660  CaO  [•  0.3307 A12O3  1.942  SiO2 

0.4005  ZnO  J 

i.o     RO 

No  analysis  of  sufficient  accuracy  was  obtained  of  a 
lead  glaze  for  this  purpose.  It  would  appear,  however, 
that  in  these,  0.2  equivalents  of  lead  oxid  substitutes 
the  equivalent  of  lime  and  that  the  silica  and  alumina  are 
higher. 

EXAMINATION  OF  THE  MATERIALS. — The  chemical 
analysis  gives  the  most  valuable  data  for  judging  both 
stoneware-  and  slip-clays.  In  case  of  the  former,  it  is 
necessary  to  calculate  from  the  analysis  the  proportions 
of  the  equivalent  of  alumina  to  silica,  in  order  to  see  if 
the  clay  is  likely  to  salt-glaze  well. 

The  chemical  formula  of  the  enduring  constituents  of 
a  slip-glaze  will  show  at  once  in  how  far  it  will  answer 
the  purpose  of  giving  a  glass  fusible  at  the  heat  of  melt- 
ing of  pyro metric  cone  eight,  or  how  it  must  be  modified 
to  meet  this  end.  Still  the  chemical  data  must  be  con- 
firmed by  practical  burning  trials  to  make  certain  that 
the  stoneware  clays  become  sufficiently  dense,  and  that 
the  slip-glaze  burns  brown  and  not  green. 


CHAPTER  IX. 

RAW  MATERIALS   OF  WHITE-WARE 
BODIES. 


various  white  wares  are  very  sel- 
dom made  in  a  "self-body,"  that 
is,  of  a  natural  white-burning  clay, 
requiring  no  artificial  admixture  to 
enhance  the  whiteness,  increase  the 
plasticity,  better  the  hardness  when  burned,  or  modify 
the  coefficient  of  expansion. 

The  manufacturer  of  such  ware  must,  therefore,  be 
well  equipped  with  appliances  for  washing  and  mixing 
the  various  ingredients  of  his  bodies  and  he  must  be 
better  endowed  with  ceramic  skill  than  the  red- ware, 
yellow- ware,  or  stoneware  potters  who  accept  as  bodies 
the  clays  furnished  by  nature,  without  modification. 

The  ingredients  of  white-ware  bodies  are  quite  the 
same  as  those  making  up  the  clays  of  the  commoner 
kinds  of  ware,  namely  clay  substance,  free  silica,  and 
fusible  minerals  or  basic  oxids,  but  they  must  be  as  pure 
as  they  are  severally  furnished  by  nature  or  as  they  can 
be  rendered  by  mechanical  means. 

The  clays  required  are  both  primary,  china-clay  or 
kaolin,  and,  secondary,  pipe  or  ball-clay  ;  the  former 
giving  whiteness,  the  latter  plasticity  to  the  bodies. 


94  THE    CHEMISTRY   OF   POTTERY. 

Kaolin  or  china-clay  occurs  in  considerable  quantities 
and  varieties  throughout  the  Appalachian  system  of  the 
United  States,  Pennsylvania,  Delaware,  Virginia,  and 
North  Carolina  marketing  large  quantities.  Most  of 
that  sold  is  selected,  graded,  and  washed  at  the  mines, 
by  the  companies  exploiting  the  same  for  the  pottery  in- 
dustry. Very  little  is- sold  in  its  natural  condition. 

In  perhaps  the  majority  of  cases,  the  kaolinized  rock 
contains  the  quartz,  undecomposed  feldspar,  and  mica, 
in  crystals  and  laminae  of  such  size  that  mere  stirring 
or  ''blunging"  of  the  mineral  in  water  is  sufficient  to 
wash  out  the  kaolin,  quite  free  from  contamination  with 
detritus  of  the  other  minerals.  It  is  recovered  by  sedi- 
mentation and  passing  the  thickened  magma  through 
filter-presses. 

Still  a  kaolin  may  carry  a  very  considerable  amount 
of  minerals  nearly  as  finely  divided  as  it  is  itself.  It 
can  be  freed  from  these  by  a  carefully  conducted  process 
of  floating,  though  there  is  in  such  a  case  no  necessity 
for  separation  at  all,  as  these  minerals  would  serve  a 
useful  purpose  in  the  pottery  body.  But  it  would  be 
necessary  to  inform  the  potter,  by  a  careful  "rational 
analysis, ' '  what  the  clay  contains  so  that  he  may  make 
allowance  for  the  ingredients  in  preparing  his  mixtures. 

An  example  of  such  a  kaolin,  containing  the  naturally 
admixed  minerals,  so  finely  divided  that  they  could  en- 
ter directly  in  a  white-ware  body  and  need  not  be  sepa- 
rated by  floating,  is  that  from  Nelson  County,  Virginia, 
the  analysis  of  which  was  given  on  page  10  (Chapter  I.). 

Another  example,  striking  in  the  very  large  amount 
of  finely  divided  quartz  it  contains,  with  very  little  feld- 


RAW   MATERIALS   OF  WHITE-WARE   BODIES.          95 

spar  remaining,  is  the  following.  It  is  unknown  from 
what  part  of  the  country  the  sample  came,  but  its 
appearance  plainly  indicated  a  primary  clay. 

The  portion  insol- 
Constituents  ol  uble  in  H2SO4  and 
the  entire  clay  Na2CO3 

Per  cent.  Per  cent. 

Silica 8i.ii  61.56 

Alumina 13.65                      0.26 

Ferric  oxid o.  13                      o.oo 

Lime 0.53                      o.  18 

Magnesia 0.90                     0.45 

Alkalies 0.91  trace 

Combined   water 3.59                      o.oo 

100.82  62.45 

RATIONAL  ANALYSIS. 

Per  cent. 

Clay  substance 35-47 

Feldspathic  mineral 1.81 

Soluble   silica 2.90 

Quartz 60.64 


100.82 
PERCENTAGE  COMPOSITION  OF  THE  CLAY  SUBSTANCE. 

Per  cent. 

Silica 46.94 

Alumina .- 38.12 

Lime 0.99 

Magnesia 1.27 

Alkalies 2.57 

Combined  water 10.12 


The  kaolins  on  the  market  do  not  differ  widely  in 
chemical  composition  from  nearly  pure  products,  five  dif- 
ferent brands  of  well-known  domestic  china-clays  analyz- 
ing as  follows : 


96  THE   CHEMISTRY   OF   POTTERY. 


Aj.vun.itig,  

4/.uu 

4°O(J 
-jQ   IT 

40.47 

7C  0, 

40.13 

45-^ 

o/Ou 

3°-6l 

OOn 

39-T7 

39-  II 

•39 

°-59 

O^Q 

IMaijnesia  

o  oA 

Alkalies    •  • 

•25 

r>  /iR 

0.30 

Combined   water. 

•25 
1347 

12.24 

13-34 

13.10 

0.97 
13.20 

Insoluble  in 
H2SO4  and  Na2CO3 

100.89 
}    2.46 

100.64 

100.53 

0.47 

100.32 
2-54 

99-99 

0.40 

Many  of  our  white-ware  potters  being  of  English  train- 
ing and  using  formulas  calling  for  English  raw  materials, 
the  following  analyses  of  three  English  china-clays  com- 
monly met  with  in  the  American  market,  are  given  for 
comparison  with  our  native  products : 


Silica  

Per  cent. 

Per  cent. 

Per  cent. 

itQ    r  T 

47.90 

^8  2Q 

47-1U 

Ferric  oxid  

^o.zy 

O/'OJ 

Ou'// 

0.44 

Magnesia  

•4o 

•5° 

Alkalies  

o  ^6 

UO4 

Combined  water  .... 

12.76 

1345 

•5° 
11.80 

Insoluble  in             ) 
H2SO4  and  Na2CO3  j 

100.90 
o.oo 

99.80 

1.24 

100.66 
8.33 

The  linear  shrinkage  of  china-clays  burned  to  the  melt- 
ing clear  of  orthoclase  feldspar,  about  the  melting-point 
of  pyrometric  cone  nine,  is  from  four  to  eight  per  cent. 

They  may  require  as  much  as  forty  grams  of  water  for 
100  grams  dry  clay,  to  make  a  mass  sufficiently  soft  to 
allow  the  Vicat  needle  to  penetrate  four  centimeters. 


f 

|S«"      *$ 

#?  > 


*4 


RAW    MATERIALS   OF   WHITE-WARE   BODIES.          97 

Nevertheless  the  mass  of  china-clay  is  of  poor  plasticity 
and  does  not,  in  the  air-dry  condition,  show  a  tensile 
strength  of  more  than  2,000  to  2,500  grams  per  square 
centimeter. 

The  commercial  value  of  a  china-clay  depends  largely 
upon  the  whiteness  of  the  body  it  will  produce.  Its  ap- 
pearance in  the  clay  state  is  absolutely  no  criterion  of 
this,  as  a  clay  which  is  very  yellow  when  unburnt  may 
become  very  white  in  the  fire.  Unfortunately  the  per- 
centage of  iron  in  china-clays,  as  shown  by  their  anal- 
yses, stands  in  no  direct  relation  to  the  tints  of  the  burned 
products.  Nor  can  a  true  comparative  estimate  of  the 
clays  be  made  when  they  are  merely  once  burned,  that 
is,  in  the  biscuit  state.  It  requires  the  covering  of  a 
thin  clear  glaze  to  bring  out  the  true  tint. 

For  the  purpose  of  comparing  the  tints  of  china-clays, 
they  must  be  made  up  into  bodies  by  the  addition  of  the 
same  proportions  of  quartz,  and  feldspar,  of  the  same 
lot,  burned  in  the  same  biscuit  fire,  and  covered  to  an 
equal  thickness  with  a  transparent  glaze,  which  had  best 
not  contain  over  one-half  equivalent  of  lead-oxide,  as  a 
glass  high  in  lead  is  of  a  yellowish  cast,  nor  should  the 
glaze  have  been  tinted  by  an  addition  of  cobalt  oxide. 

To  a  limited  extent,  silicates  of  alumina  resembling 
halloysite,  Al2O3.2SiO2.4H2O,  have  at  various  times 
found  a  local  use  in  place  of  kaolin. 

A  clay  of  this  nature  from  Lawrence  County,  Indiana, 
which  has  had,  perhaps,  a  more  extensive  pottery-appli- 
cation than  any  other  of  this  kind,  can  not  be  disintegra- 
ted by  mere  washing,  but  ground  in  water  it  gives  a  very 


98  THK   CHEMISTRY   OF   POTTERY. 

voluminous  mass  resembling  starch-paste,  and  is  pasty 
rather  than  plastic  in  character,  and  dries  down  to  a 
horn-like  body.  Burned  at  the  heat  of  running  feldspar, 
it  has  a  linear  shrinkage  of  twenty  per  cent,  from  its 
size  in  the  clay  state.  The  burned  clay  is  much  denser 
than  kaolin  and  has  a  faintly  greenish  tint. 

Partial  analyses  of  the  material  analyzed  at  intervals 
during  a  number  of  years,  contained  silica  ranging  from 
39.74  per  cent,  to  41.15  per  cent.,  and  combined  water 
from  17.21  per  cent,  to  17.78  per  cent. 

An  exhibition  specimen,  analyzed  in  full,  ran  much 
closer  to  the  proportions  of  kaolin,  though  with  the  com- 
bined water  still  high,  as  follows  : 

Per  cent. 

Silica 44-79 

Alumina 38.77 

Lime i  .00 

Magnesia 0.25 

Alkalies 0.67 

Combined   water 15-49 


100.97 
Insoluble  in  H2SO4  and  Na.,CO;, 0.98 

It  is  not  unlikely  that  more  materials  of  this  nature 
may  be  found  and  applied  to  special  uses  in  the  pottery 
art.  A  specimen  sent  to  the  writer,  as  a  kaolin  from 
Inyo  County,  California,  appears  from  the  analysis  to  be 
of  this  nature : 


RAW   MATERIALS   OF   WHITE-WARE    BODIES.          99 

Portion  insoluble 

Constituents  of  the         in  H2SO4  and 
entire  clay  Na2CO3 

Per  cent.  Per  cent. 

Silica 44-74  8.99 

Alumina 33.23  1.47 

Ferric  oxid i  .08  0.45 

Lime 0.77 

Magnesia 0.23 

Alkalies 2.24 

Titanic  oxid i.io 

Combined  water I7-5& 

100.95  10.91 

RATIONAL  ANALYSIS. 

Per  cent. 

Mineral  and  quartz  sand 10.91 

Clay  substance 90.04 

100.95 
PERCENTAGE  COMPOSITION  OF  THE 

Clay    Substance  and    Halloysite. 
Per  cent.  Per  cent. 

Silica 39.70  40.74 

Alumina 35.27  34.86 

Ferric  oxid 0.70 

Lime .         0.86 

Magnesia 0.26 

Alkalies 2.49 

Titanic  acid r 1.22 

Combined  water 19-50  24.40 


Secondary  or  plastic  clays,  known  to  potters  as  pipe- 
or  ball-clays,  occur  plentifully,  and  of  a  high  degree  of 
purity  in  a  number  of  the  tertiary  and  quarternary  expo- 
sures of  the  country ;  New  Jersey,  Florida,  western  Ken- 


100  THE   CHEMISTRY   OF   POTTERY. 

tucky  and  eastern  Missouri  furnishing  the  principal  sup- 
plies. 

They  are  necessary  additions  to  white- ware  bodies, 
because  of  their  plasticity.  Being  more  abundant  than 
kaolins,  they  are  cheaper  than  these,  but  are  also,  even 
in  the  best  varieties,  far  less  white.  Plastic  bodies  being 
in  reasonable  limits  easier  and  hence  cheaper  to  mold, 
ball-clays  are  used  in  as  large  amounts,  for  their  required 
clay  substance,  as  the  quality  of  the  ware  will  admit. 

From  their  formation  these  clays  are  very  likely  to  con- 
tain finely  divided  quartz  and  the  detritus  of  fusible  min- 
erals, hence  their  introduction  into  a  body  or  substitu- 
tion for  some  other  ball-  or  china-clay  in  a  pottery  recipe, 
should  only  be  made  with  a  clear  knowledge  of  their 
chemical  composition. 

It  is,  however,  a  striking  fact,  that  a  much  larger  pro- 
portion of  the  American  ball-clays  approach  the  compo- 
sition of  kaolinite,  than  do  those  of  Europe.  The  purest 
of  our  native  ball-clays  are  mined  in  Florida,  and  are 
being  sold  as  ' '  plastic  kaolins, ' '  a  misleading  trade  name. 

An  analysis  of  one  of  these  runs  as  follows : 

Per  cent. 

Silica 45-39 

Alumina 39. 19 

Ferric  oxid 0.45 

Lime  •  1 0.51 

Magnesia 0.29 

Alkalies 0.83 

Combined  water 14.01 

100.67 
Insoluble  in  H.2SO4  and  Na2CO3 0.87 


RAW  MATERIALS  OF  wtfiT  KARE 


ibl 


A  sample  baked  at  the  melting  heat  of  orthoclase, 
while  very  nearly  as  white  as  a  high  grade  kaolin,  had 
not  at  all  remained  porous,  but  was  dense  and  glossy, 
having  shrunk  fully  fifteen  per  cent,  in  linear  diameter. 

Next  in  purity  to  these  Florida  clays  are  those  very 
extensively  mined  in  New  Jersey.  An  analysis  of  a 
typical  one  of  these  clays  is  the  following  : 

Portion  insoluble 

Composition  of  the  in  H2SO4  and 

entire  clay  Na2CO3 

Per  cent.  Per  cent. 

Silica  .....................     46.18  3-20 

Alumina  ..................     39.08  0.39 

Ferric  oxid  ...............       i.n 

Lirne  .....................       0.42  0.05 

Magnesia  .................       0.35  0.05 

Potash  ....................       0.23  0.05 

Soda  ......................       0.28  0.21 

Combined  water  ..........     13.04  o.oo 

100.69  3.95 

Burned  at  the  melting  heat  of  orthoclase  the  clay  was 
yellowish  white  and  shrank  fourteen  per  cent. 

The  clay  is  of  good  plasticity  and  requires  51.5  grams 
of  water  for  100  grams  of  dry  clay,  in  order  to  be  pene- 
trated by  the  Vicat  needle. 

However,  its  binding  power  is  very  low.  Briquettes 
formed  of  the  plastic  clay  would  not,  in  the  dry  condi- 
tion, bear  a  strain  of  more  than  1600  grams  per  square 
centimeter. 

The  ball-clays  of  Western  Kentucky  and  Eastern 
Missouri,  while  less  pure  than  the  foregoing,  are  of  far 
greater  binding  power,  giving  them,  for  certain  pur- 


OF   POTTERY. 

poses,  much  the  preference  over  the  former.  In  fact,  be- 
cause of  the  deficiency  of  the  New  Jersey  clays,  in  this 
particular,  not  inconsiderable  quantities  of  English  plas- 
tic clays,  that  could  well  be  replaced  by  these  western 
domestic  ones,  are  imported. 

A  ball  clay  from  Jefferson  County,  Missouri,  of  estab- 
lished reputation,  has  the  composition  : 

Portions  insoluble 

in  H2SO4  and 

The  entire  clay  Na3CO3 

Per  cent.  Per  cent. 

Silica 48.5 1  2.85 

Alumina 35-i8  0.75 

Ferric  oxid 0.92 

Lime i  .01  0.06 

Magnesia 1.47  0.48 

Alkalies 2.30  0.35 

Combined  water 10.72  o.oo 

ioo.ii  4.49 

Burned  at  the  melting  of  pyrometric  cone  nine,  the 
shrinkage  of  the  clay  is  fifteen  percent.,  and  the  body  is 
not  only  dense  from  the  high  contraction  and  the  charac- 
teristic structure  of  the  plastic  clays,  but  is  vitrified  as  a 
direct  result  of  the  high  percentage  of  fluxing  oxides  it 
contains. 

A  plastic  clay  brought  into  the  Market  from  Galloway 
County,  Kentucky,  gave  the  following  analysis  : 

The  portion  insoluble 

in  H2SO4  and 

The  entire  clay  Na2CO3 

Percent.  Per  cent. 

Silica 59.83  26.02 

Alumina 27.80  0.29 

Ferric  oxid 0.83  0.07 


RAW   MATERIALS  OF   WHITE-WARE   BODIES.        103 

The  portion  insoluble 

in  H2SO4  and 

The  entire  clay  Na2CO3 

Per  cent.  Per  cent. 

Lime 0.15  0.05 

Magnesia 0.24  trace 

Alkalies 0.82  0.46 

Combined  water 10.42  o.oo 


100.09  26.89 

RATIONAL,  ANALYSIS. 

Per  cent. 

Clay  substance 73.20 

Feldspathic  detritus 1 .92 

Quartz 24.97 


100.09 
PERCENTAGE  COMPOSITION  OF  THE  CI,AY  SUBSTANCE. 

Per  cent. 

Silica 46. 19 

Alumina 37-59 

Ferric  oxid i  .03 

Lime 0.13 

Magnesia 0.32 

Alkalies 0.49 

Combined  water 14.25 

At  the  melting  heat  of  feldspar  the  clay  gives  a  dense 
body,  but  having  still  some  suction  when  applied  to  the 
moist  tongue  ;  it  is  of  a  yellowish- white  color  and  has  a 
shrinkage  of  ten  per  cent. 

With  the  exception  of  the  Florida  and  some  of  the 
New  Jersey  clays,  ball  clays  are  put  on  the  market  un- 
washed. 

They  almost  invariably  contain  iron  pyrites,  some- 
times in  large  nodules. 


104  THE    CHEMISTRY   OF   POTTKRY. 

Frequently  the  more  plastic  English  and  Kentucky 
clays  contain  sufficient  organic  matter  to  appear  quite 
black  in  their  raw  state.  This  may  be  determined 
analytically  with  sufficient  accuracy,  by  dissolving  the 
clay  in  hydrofluoric  and  hydrochloric  acids  on  the 
water-bath,  and  filtering  off  the  organic  flocks  on  a  tared 
paper,  where,  after  drying,  it  is  weighed.  By  this 
method  these  clays  have  often  yielded  the  writer  as  much 
as  four  per  cent,  of  organic  impurity. 

Like  the  china-clays  the  ball-clays  should  be  exam- 
ined comparatively  for  tint,  working  them  up  into  bodies 
with  pure  flint  and  feldspar  and  covering  the  biscuit 
pieces  with  a  clear  glaze,  in  the  manner  already  de- 
scribed. 

The  uncombined  silica  of  pottery  bodies,  in  so  far  as 
it  is  not  furnished  by  the  clays  in  compounding  the 
same,  is  added  in  the  form  of  finely  ground  flint  or 
quartz. 

French  flints  are  cheaply  imported  as  ship  ballast  and 
used  to  a  considerable  extent,  though  native  flint  of  a 
high  degree  of  purity  is  obtainable  and  also  largely 
used.  Flint  is  roasted  previous  to  grinding,  becoming 
friable  through  loss  of  its  organic  matter.  It  should 
burn  perfectly  white,  though  a  faint  pinkish  cast  is  often 
not  objectionable,  as  the  feldspathic  flux  of  white-ware 
bodies  takes  up  the  small  amount  of  ferric  oxid  produc- 
ing it,  giving  a  perfectly  white  sinter. 

A  native  flint  of  average  commercial  quality,  put  on 
the  market  by  a  spar-miller  of  Trenton,  N.  J.,  has  the 
composition  : 


RAW   MATERIALS   OF   WHITE-WARE   BODIES.        105 

Per  cent. 

Alumina 0.33 

Ferric  oxid 0.27 

Lime o.  13 

Magnesia 0.09 

Alkalies o.  1 1 


Total   impurities 0.93 

Moisture 0.24 

Silica  (by  difference) 98.83 

100.00 

Heated  to  the  melting  heat  of  orthoclase,  it  remained 
snowy  white  and  did  not,  in  the  least,  sinter  together. 

A  quartz  sand,  of  high  purity,  mined  in  L/aSalle 
County,  Illinois,  which  is  ground  for  pottery  purposes, 
has  the  following  composition  : 

Per  cent. 

Alumina o.  155 

Ferric  oxid 0.069 

Lime 0.026 

Magnesia 0.013 

Alkalies o.  1 12 

Total  impurities 0.375 

Moisture 0.070 

Silica  (by  difference) 99-555 

100.00 

There  are,  in  the  country,  deposits  of  silica  contain- 
ing but  small  amounts  of  accompanying  clay  and  min- 
eral detritus,  that  are  very  finely  divided,  requiring 
practically  no  grinding  and  burning  very  white  indeed. 
Such  materials  are,  as  yet,  not  utilized  for  pottery  pur- 
poses, though  they  would  furnish  an  excellent  and  cheap 


io6 


THB    CHKMISTRY   OF   POTTERY. 


substitute  for  the  artificially  ground  flint,  allowance  be- 
ing made  in  using  them,  for  the  clay  and  fusible  min- 
erals which  they  contain.  Being  powdered  by  natural 
agencies,  possibly  by  precipitation,  they  seem  to  com- 
bine more  intimately  with  the  clays  of  a  mass,  making 
often  a  more  plastic  and  perfectly  knit  body,  than  is  ob- 
tained with  silica  reduced  by  grinding. 

A  material  of  this  character,  from  along  the  Cumber- 
land river  in  Tennessee,  analyzed  : 

The  portion  in- 

The  entire  soluble  in  H2SO4 

sample  •                and  Na2CO3 

Per  cent.  Per  cent. 

Silica 85.80  65.17 

Alumina 9.75  0.50 

Ferric  oxid 0.46  0.04 

Lime 0.20  0.08 

Magnesia 0.23  0.07 

Alkalies 0.98  0.35 

Combined  water 3.07  o.oo 

100.49  66.21 

RATIONAL,  ANALYSIS. 

Per  cent. 

Clay  substance 32.74 

Feldspathic  detritus 2.94 

Quartz 63.27 

Soluble  silica   i  .54 

100.49 
PERCENTAGE;  COMPOSITION  OF  THE  CLAY  SUBSTANCE. 

Per  cent. 

Silica 58.31 

Alumina •  •  •  •     28.25 

Ferric  oxid 1.28 


RAW   MATERIALS   OF   WHITE-WARE    BODIES.        IOy 

Per  cent. 

Lime • 0.36 

Magnesia 0.48 

Alkalies 1.92 

Combined  water 9.37 


Baked  at  the  melting-point  of  feldspar  the  material 
has  a  shrinkage  of  one  and  a  half  per  cent.,  and  when  it 
is  still  porous  is  much  denser  than  a  mixture  of  a  flint 
and  ball-clay  according  to  its  rational  analysis  would 
become,  as  a  steel  point  marks  but  does  not  scratch  it. 

Still  more  striking  is  a  material  of  this  character  from 
Galloway  County,  Kentucky.  Quite  friable  in  its 
natural  state,  it  hardens  somewhat  when  immersed  in 
water,  like  a  weak  plaster.  This,  however,  is  very 
easily  ground  to  a  mass  which  no  longer  sets.  Its 
analysis  is  : 

The  portion  in- 

The  entire  soluble  in  H2SO4 

material  and  Na2CO3 

Per  cent.  Per  cent. 

Silica 90.49  77.00 

Alumina 5.45  0.50 

Ferric  oxid 0.39  0.02 

Lime 0.23  0.03 

Magnesia 0.30  0.07 

Alkalies 1.74  0.33 

Combined  water i  .64  o.oo 


100.24  77.95 

RATIONAL  ANALYSIS. 

Per  cent. 

Clay  substance 18.70 

Feldspathic  detritus 2.71 

Quartz 75.24 

Soluble  silica 3.59 

100.24 


108  THE   CHEMISTRY   OF   POTTERY. 

PERCENTAGE  COMPOSITION  OF  THE  CI,AY  SUBSTANCE. 

Per  cent. 
Silica 53.0 

Alumina 26.4 

Ferric  oxid •  •  •  • 2.0 

Lime i.i 

Magnesia 1.2 

Alkalies 7.5 

Combined  water 8.8 


100.0 

Burned  at  the  heat  of  running  spar,  the  material 
shrinks  but  one-half  per  cent,  and  is  very  white.  It  re- 
mains more  porous  than  the  material  just  described  and 
can  just  be  scratched  with  a  knife  blade. 

Of  the  fluxes  used  in  the  bodies  of  white  ware,  feld- 
spar is  the  most  important.  It  is  put  upon  the  market 
by  a  number  of  spar-millers  of  Connecticut,  New  Jersey, 
Ohio,  and  Pennsylvania  of  satisfactory  purity  and  of 
different  fusibilities. 

A  good  commercial  potash  feldspar  gave  the  follow- 
ing analysis : 

Per  cent. 

Silica 65.85 

Alumina J9-32 

Ferric  oxid 0.24 

Lime 0.56 

Magnesia 0.08 

Alkalies1 14. 10 

100.15 

A  commercial  soda-lime  feldspar,  materially  more 
fusible  than  the  former,  being  sold  as  a  ' '  soft  spar, ' ' 
analyzed  : 


1  Combining  weight,  45.9. 


RAW   MATERIALS   OF   WHITE-WARE   BODIES.        IOQ 

Per  cent. 

Silica 66.81 

Alumina 21.09 

Ferric  oxid o.  13 

Lime 2.03 

Magnesia o.  10 

Alkalies1 9.64 


99.80 

Often  the  quartz  veins  of  a  spar-bed  may  be  difficult 
to  remove  or  the  workmen  are  careless  in  picking  over 
the  mineral,  causing  more  or  less  variation  of  the  com- 
mercial product,  against  which  the  consumer  must  be  on 
his  guard. 

Again,  it  may  occur,  that  the  more  quartzose  por- 
tions of  a  soda-lime  spar^are  ground  separately  and  sold 
as  ' '  hard  or  potash  spar, ' '  as  was  the  case  with  a  lot  of 
which  the  following  is  an  analysis  : 

Per  cent. 

Silica 68.82 

Alumina 19-75 

Ferric  oxid o.  16 

Lime i  .64 

Magnesia o.  1 7 

Alkalies* • 9.15 


99.69 

It  is  of  about  the  same  fusibility  as  the  true  potash- 
spar  shown  in  the  first  analysis,  and  would  deceive  one 
in  the  mere  kiln-fusibility  test,  which  is  all  that  potters 
usually  make  to  verify  the  quality  of  the  material. 

1  Combining  weight,  33.5. 

2  Combining  weight,  36.4. 


110  THK    CHEMISTRY   OF   POTTERY. 

The  practical  bearing  of  the  difference  in  composition 
of  these  two  feldspars  of  equal  fusibilities,  is  demon- 
strated in  their  varying  coefficients  of  expansion — a 
serious  difference  if  one  were  substituted  for  the  other  in 
a  body  having  to  carry  a  certain  glaze. 

This  difference  is  practically  shown  in  the  fact  that 
the  potash  feldspar  melted  in  a  thick  layer  upon  a  pot- 
tery body,  cracks  off  of  one  upon  which  the  siliceous 
soda-lime  feldspar  remains  immovably  fixed. 
.  Cornwall  stone,  a  partly  decomposed  granite,  mined 
in  Cornwall,  England,  is  used  by  English  potters  as 
their  principal  pottery  flux  and  also  finds  considerable 
application  in  the  United  States.  All  that  is  used  here 
is  imported,  no  material  resembling  it  having  as  yet 
been  commercially  developed  within  our  borders. 

An  average  sample  of  a  good  quality  of  this  material 
has  the  following  composition  : 

The  portion  in- 

The  entire  soluble  in  H2SO4 

material  and  Na2CO3 

Per  cent.  Per  cent. 

Silica 73.57  57.69 

Alumina 16.47  4-7° 

Ferric  oxid 0.27  0.30 

Lime 1.17  o.io 

Magnesia 0.21  0.12 

Alkalies 5.84  3.50 

Combined  water 2.45  o.oo 

99.98  66.41 

Combining  weights  of  the 

alkalies 44.6  38.4 


RAW   MATERIALS   OF   WHITE-WARE   BODIES.        Ill 
RATIONAL  ANALYSIS. 

Per  cent. 

Clay  substance  and  mica 33-57 

Feldspar 25.31 

Quartz 4i-io 

99.98 
PERCENTAGE  COMPOSITION  OF  THE 

Clay  Substance  and  Mica          Feldspar 
Per  cent.  Per  cent. 

Silica 47-27  65-55 

Alumina 35-Q4  18.57 

Ferric  oxid o.oo  i .  18 

Ivime 3.18  0.40 

Magnesia 0.26  0.47 

Alkalies 6.96  13.83 

Combined  water 7.29  o.oo 

100.00  100.00 

The  figures  show  that  the  kaolinizing  decomposition 
of  the  rock  has  proceeded  to  but  a  limited  extent,  the 
"  clay  substance,"  as  in  this  sample,  consisting  in  the 
main  of  mica.  This  is  further  proven  by  the  constant 
presence  of  fluorine,  which,  though  it  has  been  ignored 
as  a  separate  element  in  the  above  analysis,  has  been 
found  present  to  the  extent  of  1.66  per  cent. 

It  may  be  justified,  in  the  case  of  this  material,  in 
which  mica  plays  nearly  as  important  a  part  as  the  feld- 
spar as  fluxing  constituent,  to  give  it  a  separate  place  in 
the  rational  analysis,  for  the  better  guidance  of  the  pot- 
ter. 

Cornish  stone  is  by  no  means  as  uniform  in  character 
and  composition  as  potters  generally  believe. 

The  portion  insoluble  in  sulfuric  acid  and  sodium 
carbonate  solution  is  in  many  cases  markedly  greater  in 


112  THE    CHEMISTRY   OF    POTTERY. 

alumina  than  in  that  of  which  the  analysis  has  been 
given  and  not  infrequently  the  silica  is  either  largely 
soluble  in  the  sodium  carbonate  solution  or  is  more 
readily  made  so  by  the  action  of  sulfuric  acid  than 
quartz  commonly  is. 

A  sample  showing  both  of  these  peculiarities  analyzed 
as  follows : 

The  portion  in- 

The  entire  soluble  in  H2SO4 

material  and  Na2CO3 

Per  cent.  Per  cent. 

Silica 72.99  42'72 

Alumina 17*58  7.83 

Ferric  oxid 0.15  o.io 

Lime 1.25  0.71 

Magnesia 0.37  0.19 

Alkalies 6.20  4.31 

Combined  water 1.77  o.oo 

100.31  55.86 

RATIONAL,  ANALYSIS. 

Per  cent. 

Clay  substance,  mica,  and  soluble  silica 44-45 

Feldspar 40.68 

Quartz 15.18 

100.31 
PERCENTAGE  COMPOSITION  OF  THE 

Clay  Substance  Etcetera          Feldspar 
Per  cent.  Per  cent. 

Silica 68. 10  67.68 

Alumina 21.94  19-24 

Ferric  oxid o.ii  0.25 

Lime 1.21  1.75 

Magnesia 0.41  0.47 

Alkalies 4. 25  10.61 

Combined  water 3.98  o.oo 


RAW   MATERIALS   OF  WHITE-WARE    BODIES.        113 

The  sum  of  the  alkali  and  combined  water  in  this 
* '  clay  substance ' '  falls  far  short  of  what  would  be  de- 
manded by  a  mixture  of  mica  and  pure  clay ;  while  on  sub- 
tracting the  excess  of  silica,  assuming  it  as  uncombined 
but  soluble  in  sodium  carbonate  solution,  and  recalcu- 
lating the  residue  on  a  percentage  basis,  they  assume 
the  proper  proportion.  This  would  make  the  rational 
analysis  : 

Per  cent 
Clay  substance  and  mica 25. 71 

Feldspar 40.68 

Quartz 15.  iS 

Soluble  silica 


100.31 

The  percentage  composition  of  the  clay  substance 
and  mica  then  is  as  follows : 

Per  cent. 

Silica 44.85 

Alumina 37-93 

Ferric  oxid o.  19 

L/ime 2. 10 

Magnesia 0.70 

Alkalies 7.35 

Combined  water 6.88 

Direct  determinations  of  soluble  silica  in  a  number  of 
such  specimens  failed  to  yield  anything  like  the  required 
amount,  leading  to  the  conclusion  that  sulfuric  acid  may 
in  many  cases  have  more  action  on  some  form  of  silica, 
in  this  mineral,  as  well  as  fjn  other  clays,  than  that 
already  pointed  out  in  the  case  of  quartz,  rendering  it  in 
greater  measure  soluble  in  sodium  carbonate  solution. 


114  THE   CHEMISTRY   OF   POTTERY. 

But  more  important  than  the  difference  in  the  physical 
character  of  the  contained  minerals  or  a  variation  in  the 
apportionment  of  the  elements  to  the  different  mineral 
groups,  is  the  variation  in  ultimate  chemical  composi- 
tion of  Cornish  stone,  particularly  in  the  proportion  of 
the  alkalies,  as  in  the  following : 

Per  cent.  Per  cent. 

Silica 74.55  73.77 

Alumina 17-37  16.05 

Ferric  oxid 0.26  0.23 

Lime 1.68  1.14 

Magnesia 0.54  0.22 

Alkalies 3.68  7.52 

Combined   water 2.04                   .        1.78 


100.12  100.71 

While  Cornish  stone  has  distinct  uses  determined  by 
its  physical  properties,  it  could,  in  the  vast  majority  of 
the  cases  of  its  application  in  the  United  States,  be  more 
cheaply  and  safely  substituted  by  equivalent  amounts  of 
native  feldspar,  kaolin,  and  quartz,  which  run  much 
more  uniform  in  quality. 

An  anomalous  material,  white,  of  fine  grain,  free 
from  crystals  of  quartz  and  feldspar,  and  flakes  of  mica, 
resembling  a  kaolin  in  appearance,  was  obtained  by  the 
writer  from  Fayette  County,  Texas,  where  it  is  said  to 
occur  in  large  deposit.  It  resembles  Cornish  stone  in 
composition  more  nearly  than  any  native  material  which 
has  thus  far  fallen  into  the  writer's  hands. 

Its  analysis  is  as  follows : 


HAW   MATERIALS   OF  WHITE-WARE   BODIES.        115 


The  entire 

substance 

Per  cent. 


The  portion  in- 
soluble in  H2SO4 
and  Na2C03 
Per  cent. 


Silica 68.88  43.60 

Alumina 16.77  7-91 

Ferric  oxid 0.83  0.32 

Ivime 0.99  0.26 

Magnesia 0.17  0.18 

Alkalies 6.77  2.56 

Combined  water 5.79  o.oo 

Sulfuric  anhydrid- 0.42  0,00 

100.62  54.76 

Soluble   silica   (directly 

determined) 5,23 

RATIONAL,  ANALYSIS. 

Per  cent- 
Clay  substance 39.83 

Feldspathic  detritus 40. 28 

Quartz 14.55 

Soluble  silica 5.23 

Calcium  sulfate 0.74 

100.63 

PERCENTAGE  COMPOSITION  OF  THE  CI^AY  SUBSTANCE. 

Per  cent 

Silica 50.33 

Alumina 23.52 

Ivime 1.03 

Alkalies - 10.57 

Combined  water 14.54 

The  combined  water,  in  this  case,  seems  to  belong 
largely  to  the  soluble  silica,  which  was  hydrated  so  that 
a  portion  of  it  could  be  extracted  with  distilled  water 


Il6  THE   CHEMISTRY   OF   POTTERY. 

alone  ;  but  as  there  was  no  way  of  apportioning  it,  it 
was  counted  in  with  the  "  clay  substance." 

This  material  was  substituted  for  Cornish  stone,  both 
in  body  and  glaze  trials,  with  entirely  concordant  re- 
sults. 

Calcium  carbonate,  which  is  much  used  by  the 
continental  potters,  finds  but  very  limited  application 
with  us  as  a  body-flux. 

Commercial  whiting  is  a  pure  form  of  this  material, 
as  shown  in  its  analysis  in  a  previous  chapter,  though 
a  higher  grade  is  put  upon  the  market  for  the  use  of 
potters,  under  the  name  "  Paris  white/' 

Should  the  manufacture  of  a  ware  covered  with  tin 
enamels,  make  a  body  containing  a  large  proportion  of 
calcium  carbonate  necessary,  such  as  that  used  for  mak- 
ing the  tile  of  the  German  and  Swiss  "Kacheloefen,"  a 
cheap  and  reasonably  pure  form  of  this  material  would 
be  found  in  the  fresh  water  marls  of  northern  Ohio  and 
Indiana,  some  of  which  are  now  exploited  in  the  produc- 
tion of  Portland  cement. 

Samples  of  these  were  found  of  the  following  compo- 
sition : 

Per  cent.  Per  cent. 

Silica 1.16  10.45 

Alumina trace  4.20 

Ferric  oxid 0.05  trace 

Calcium  carbonate 94-7$  79-35 

Magnesium  carbonate 0.19  trace 

Organic  matter  and  loss. .  •       3.82  6.00 

100.00  100.00 


CHAPTER  X. 

WHITE  GRANITE  AND  CREAM-COLORED 
WARE. 


HE  bulk  of  the  dishes  used  for  table  ser- 
vice and  all  of  those  for  the  toilet  and  for 
modern  sanitary  plumbing,  belong  to  the 
category  of  "Granite  and  C.  C."  ware. 
White  graniteware  or  ironstone  china 
differs  only  from  "  C.  C."  that  is  cream- 
colored  ware  and  ivory  ware,  in  being  made  with  a  larger 
proportion  and  better  quality  of  kaolin,  so  as  to  be  as 
white  in  color  as  possible ;  the  whiteness  being  generally 
heightened  by  the  neutralizing  of  any  faintly  yellowish 
cast,  through  the  addition  of  cobalt-blue  to  the  body  and 
covering  the  same  with  glazes  in  which,  among  the  basic 
constituents,  the  alkalies  and  alkaline  earths  at  least 
equal  or  exceed  the  equivalent  of  lead.  Beyond  this, 
there  is  no  technical  distinction  between  the  wares  repre- 
sented by  these  and  some  other  trade  names.  Naturally 
those  which  are  made  with  less  and  cheaper  kaolin, 
having  a  larger  proportion  of  plastic  clay,  are  more  easily 
fashioned,  and  when  covered  with  a  more  plumbiferous 
and  hence  more  fusible  glaze,  are  commoner  and  cheaper. 
The  bodies  are  made  mainly  from  the  materials  dis- 
cussed in  the  previous  chapter,  their  general  composition 
not  varying  widely  from  the  following : 


Il8  THE    CHEMISTRY  OF   POTTERY, 

50  to  60  per  cent,  clay  substance, 
38  to  32  per  cent,  quartz, 
12  to    8  per  cent,  feldspar. 

Where  Cornwall-stone  is  used  as  the  flux,  its  propor- 
tion is,  of  course,  larger  than  that  of  the  feldspar  given 
above  and  is  about  such  that  its  mica  and  feldspathic 
mineral  equal  in  amount  the  required  feldspar,  the  addi- 
tional quartz  and  clay  substance  replacing  their  weight 
of  these  in  the  formula. 

The  clay  substance  is  made  up  of  kaolin  and  plastic 
clay.  In  the  whiter  varieties  there  may  be  two-thirds  of 
the  former  to  one-third  of  the  latter,  while  in  the  more 
tinted  bodies  the  proportions  may  be  reversed ;  but  in 
this  there  is  no  rule  technically  imperative,  for  the  tint 
of  body  desired,  the  relative  plasticity  of  the  clays,  and 
cost  thereof  determine  the  mixture. 

If,  as  is  frequently  the  case,  a  plastic  clay  be  used, 
bearing  a  considerable  proportion  of  quartz,  feldspathic 
mineral,  or  a  clay  substance  rich  in  alkalies  and  alkaline 
earths,  the  amounts  of  added  quartz  and  flux  are  propor- 
tionately less  in  the  body  made  with  it. 

The  bodies  in  use  have  grown  up  from  the  most  hap- 
hazard of  empirical  trials,  but  a  recalculation  of  the  mix- 
tures, with  reference  to  the  composition  of  the  ingre- 
dients, will  show  most  of  them  to  fall  within  the  limits 
of  the  general  formula  given.  If  this  be  used  as  the 
starting  point,  comparatively  few  empiric  trials  would 
have  to  be  made  to  attain  a  body  of  any  desired  charac- 
ter, with  materials  of  known  composition. 

The  materials  for  a  body  are  weighed  off  in  the  propor- 
tion required,  and  mixed  in  the  wet  way,  in  the  '  'blunger" 


WHITE  GRANITE  AND  CREAM-COLORED  WARE.      119 

already  referred  to  in  the  washing  of  yellow- ware  clays. 

Errors  in  the  proportions  of  the  constituents  entering 
into  the  body  often  occur  from  the  fact  that  potters ,  as  a  rule , 
are  not  accustomed  to  determine  and  allow  for  the  vary- 
ing amounts  of  moisture,  which  the  materials  may  con- 
tain. Thus  the  powdered  flint  or  quartz  will  contain  from 
one  to  four  per  cent,  feldspar,  from  two  to  five  per  cent, 
china-clays,  and  Cornish-stone  from  two  to  ten  per  cent., 
and  the  various  plastic  or  ball-clays  will  seldom  contain 
less  than  ten  per  cent.,  and  they  may  often  contain  as 
much  as  twenty-five  per  cent,  uncombined  water. 

Some  of  the  highly  plastic  English  ball-clays,  as  also 
those  from  western  Kentucky,  may  show  in  different  ship- 
ments considerable  variation  in  the  amount  of  contained 
carbonaceous  matter.  This  should  be  determined  and 
allowed  for,  as  well  as  any  moisture.  Systematic  mois- 
ture determinations  and  recalculations  of  formulas  in  ac- 
cord with  the  results  would  easily  obviate  annoying  va- 
riations in  the  bodies. 

After  the  mixture  has  been  stirred  or  "blunged"  to  a 
homogeneous  slip,  this  is  passed  through  a  revolving, 
shaking  or  vibrating  sieve  covered  with  a  No.  12  silk 
bolting-cloth  or  a  wire  cloth  having  one  hundred  and 
twenty  meshes  to  the  linear  inch.  By  this  means,  such 
accidental  impurities  as  chips,  grain  and  cinders,  which 
get  into  the  materials  from  the  railway  cars,  are  removed, 
as  also  sand  and  the  particularly  troublesome  small  gran- 
ules of  iron  pyrites  of  the  ball-clays. 

Here,  however,  is  also  a  point  where  the  composition 
of  the  body  may  materially  change.  If  the  slip  be  too 


120  THE   CHEMISTRY  OF   POTTERY. 

thick,  a  considerable  quantity  of  the  plastic  clay  will  be 
taken  out  of  the  mixture ;  but  the  most  important  dan- 
ger arises  from  insufficient  grinding  of  the  fluxes,  an  ap- 
preciable percentage  of  which  may  not  be  fine  enough  to 
pass  the  sieve.  This  is  particularly  the  case  since  the 
introduction  of  dry-  in  place  of  wet-grinding  by  the  spar- 
millers  r  and  is  most  to  be  looked  for  in  the  case  of  Corn- 
ish-stone, the  tough  and  flexible  crystals  of  mica,  of  which 
resist  reduction  in  pulverizing  cylinders  much  more  than 
they  do  the  crushing  and  tearing  action  of  the  buhr- stones 
in  the  wet-drag  mills.  Cornish-stone  has  been  found  on 
the  market  leaving  as  much  as  fifty  per  cent,  residue  on 
a  No.  12  silk  bolting  cloth,,  and  it  is  seldom  so  fine  as  not 
to  leave  five  per  cent. 

As  already  stated,  it  is  customary,  in  the  case  of  the 
whiter  bodies,  to  add  a  small  amount  of  cobalt,  to  neu- 
tralize any  faintly  yellowish  cast,  which  they  may  show. 
This  is  usually  added  in  the  form  of  "  blue  calx,"  a  kind 
of  Thenard's  blue,  made  by  roasting  a  mixture  of  cobalt 
oxid  with  a  china-clay  and  quartz  and  grinding  the  re- 
sulting mass  to  great  fineness  in  water. 

As  the  most  productive  whitening  effect  and  the 
greatest  homogeneity  of  color  is  obtained  by  the  most 
perfect  division  and  distribution  of  the  cobalt,  it  is  bet- 
ter to  effect  this  by  chemical  precipitation  than  by  me- 
chanical grinding.  For  this  purpose,  all  that  need  be 
done,  is  to  add  to  the  slip  a  solution  of  the  necessary 
quantity  of  a  soluble  cobalt  salt.  The  natural  alkalinity 
of  most  waters  will  be  sufficient  to  precipitate  the  cobalt 
perfectly,  lodging  it  on  every  particle  of  the  charge  in 


WHITE  GRANITE  AND  CREAM-COLORED  WARE.      121 

even  distribution  without  spots  or  specks  in  the  finished 
body. 

Formerly,  the  useless  precaution  was  taken  to  pass 
the  slip  through  a  trough  in  which  a  row  of  magnets 
was  suspended,  with  the  idea  that  iron  was  to  be  re- 
moved from  the  clay  in  this  manner  ;  this  belief  is  now 
practically  past  and  the  prejudice  against  the  more  con- 
venient wrought-iron  blunging  tubs  and  cast  iron  filter- 
presses  is  also  fast  disappearing. 

The  bod)^  mixed,  sifted,  and  thickened,  is  fashioned 
into  ware,  which  is  dried  and  placed  in  the  kiln  for  the 
biscuit  fire. 

The  temperature  at  which  this  is  finished  ranges  from 
the  melting  of  pyrometric  cones  eight  to  ten  ;  the  em- 
piric trials  for  judging  the  fire  being  biscuit  rings  coated 
with  Albany  slip-clay  and  with  feldspar. 

The  successive  changes  in  appearance  of  the  former 
material  indicate  to  the  experienced  eye,  the  progress  of 
the  fire,  while  the  melting  of  the  feldspar  determines  the 
finish  of  the  burn. 

The  properly  burned  biscuit-ware  should  be  of  such 
hardness  that  it  is  not  possible  to  scratch  it  with  a  steel 
point ;  yet  it  must  be  of  uniform  porosity,  it  being  par- 
ticularly undesirable  for  the  thin  edges  and  raised  points 
of  the  modeling  on  the  ware  to  be  so  dense  as  not  to  ad- 
here when  touched  to  the  tongue. 

A  moderately  and  uniformly  porous  body,  on  being 
immersed  in  the  creamy  liquid  of  the  glaze  materials, 
ground  and  suspended  in  water,  absorbs  the  latter, 
which  deposits  a  uniform  coat  of  its  suspended  solids  on 


122  THE   CHEMISTRY   OF   POTTERY. 

the  surface  of  the  piece.  Where,  however,  this  is  too 
dense  to  take  the  water  up,  its  solids  are  washed  off,  on 
the  withdrawal  of  the  piece  from  the  dipping  tub,  and 
after  passing  through  the  glost  fire  these  parts  appear 
dry  and  rough. 

The  best  glazes,  for  these  wares,  approach  a  formula 
of  the  following  character  : 

0.25  KNaO) 

0.50  CaO         0.3  A1A 

0.25  PbO      ) 

i.o  RO  3.5 

A  glost-kiln  heat  approaching  the  melting-point  of  gold 
is  usually  given,  at  which  the  glaze  not  only  runs  perfect- 
ly bright  and  smooth  but  eats  itself  also  slightly  into  the 
surface  of  the  body,  taking  up  additionally  from  the  lat- 
ter a  small  amount  of  silica  and  alumina.  Thus  thor- 
oughly burned,  the  glaze  should  have  the  coefficient 
of  expansion  of  a  body  of  a  formula  lying  within  the 
limits  of  that  already  given  and  burned  to  proper  bis- 
cuit-hardness. It  is  also  sufficiently  bright  and  of  such 
hardness  as  not  to  be  scratched  by  the  common  table 
cutlery. 

Deviations  from  the  formula  given,,  in  the  direction  of 
an  increase  of  lead  oxid  at  expense  of  the  lime,  and  of 
boracic  acid  at  expense  of  the  silica,  and  lessening  of  the 
alumina  and  silica  occur  in  many  degrees,  resulting  in 
relatively  more  brilliant  and  easily  fusible  glazes,  but 
these  are  also  proportionately  less  white,  more  easily 
scratched,  and  of  less  range  between  the  crazing  and 
shivering  points. 


WHITE  GRANITE  AND  CREAM-COI/)RED  WARE.      123 

Brilliance  is  however  a  consideration  in  the  sale  of 
ware  and  most  glazes  will  be  found  rather  higher  in 
lead  and  boracic  acid  than  that  of  the  formula  given. 

This  glaze  can,  of  course,  be  made  in  a  variety  of 
ways,  according  to  the  materials  at  hand,  though  the 
following  will  serve  as  a  simple  example. 

Because  of  the  alkali  and  boracic  acid  a  frit  rendering 
these  insoluble  must  first  be  made.  This  is  simply  done 
by  melting  together 

0.25  equivalent  of  powdered  borax,  47.8  parts  ; 
0.5  "  "  boracic  acid,  31.0     " 

0.25  "  "  feldspar,  69.6     " 

0.5  "  "  whiting,  25.0     " 

0.5  "  "  quartz,  15.0     " 

the  chemical  formula  of  this  frit  being 
o.25Na2O  2  o 

0.25  K20       o.25Al203     ' 
0.5    CaO 


i.o  RO 

and  its  combining  weight  141.4. 

For  the  glaze  it  is  then  necessary  to  grind  together 

0.5      equivalent  of  the  frit,  70.7  parts. 

0.25  "  "          whiting,  12.5     " 

0.25  "  "          white  lead,  32.9     " 

0.175          "  "          china-clay,'  22.7     " 

1.65  equivalents  "          quartz  49.5     " 

As  in  the  mechanical  application  of  the  glaze  mixture 

in  a  uniform  layer  to  the  surface  of  the  ware  by  dipping 

the  biscuit  pieces  in  the  glaze,  it  is  important  that  its 

constituents,    which   differ   widely  in   specific   gravity, 

•  must  remain  uniformly  mixed   and   suspended   in   the 


124  THE   CHEMISTRY   OF   POTTERY. 

water  for  a  considerable  time,  the  proportion  of  clay  in 
the  glaze  mixture,  which  helps  this  suspension,  should 
not  be  too  small.  It  is  best,  therefore,  not  to  introduce 
any  clay  into  the  frit,  but  to  reserve  it  unburned  for  the 
glaze  mixture  itself. 

Where  the  amount  of  alumina  required  by  the  glaze 
is  not  large  it  is  often  best  not  to  introduce  feldspar,  in 
order  to  have  all  the  alumina  available  for  introduction 
as  clay,  because  of  its  mechanical  use  in  keeping  the 
glaze  materials  afloat. 

The  frit  is  often  melted  in  saggars  in  the  glaze-kiln. 
In  order  to  prevent  its  sticking  to  the  same,  it  is  com- 
mon to  pour  out  the  inner  surface  of  the  saggar  with  a 
thick  milk  of  flint  and  water,  leaving  on  the  bottom  and 
the  walls  a  coating  of  flint  from  one-eighth  to  a  quarter 
of  an  inch  thick.  Into  the  saggar  thus  coated  the  frit 
mixture  is  tightly  packed.  After  baking  in  the  glost- 
fire  the  saggar  is  easily  broken  away  from  the  lump  of 
glass  which  it  contains,  and  the  excess  of  flint  adhering 
to  it  is  chiseled  off.  Any  remaining  too  firm  for  removal 
is  generally  too  insignificant  in  amount  to  affect  the 
composition  of  the  product. 

Previous  to  its  introduction  in  a  mill  for  fine  grinding, 
the  frit  should  be  crushed  small  on  a  buhrpaii  under 
buhrstone  runners.  Where  it  is  necessary  to  do  this  in 
an  iron  mortar  or  between  the  jaws  of  a  rock-  or  ore- 
breaker,  the  crushed  glass  must  be  well  freed  from  the 
not  inconsiderable  amounts  of  fine  metallic  iron,  with 
which  it  becomes  contaminated,  with  a  large  magnet 
well  cleaned  of  rust. 


WHITE  GRANITE  AND  CREAM-COLORED  WARE.      125 

The  mills  used  for  grinding  the  frit  fine  are  buhrstone 
drag-mills  and  porcelain-lined  cylinders  filled,  with  Ice- 
land flint  pebbles.  The  grinding  is  done  in  water. 

Far  better  than  melting  the  frit  in  saggars,  is  to  melt 
it  on  the  sloping  hearth  of  a  special  furnace,  from  which 
it  can  be  run  through  a  hole  at  its  lowest  point. 

By  this  means  the  melting  is  much  more  thorough. 
The  frit  is  removed  from  further  action  of  the  fire  as  soon 
as  it  is  liquid  enough  to  run  from  the  kiln  and  it  is  at 
once  cooled  and  broken  so  fine  by  falling  into  water 
that  it  can  be  introduced  into  the  fine  mills  at  once. 

The  gradual  and  long  fire  of  the  glost-kiln  is  very  lia- 
ble indeed  to  volatilize  alkali  and  boracic  acid  from  the 
mixture  before  they  are  thoroughly  combined  into  glass, 
and,  furthermore,  the  slow  cooling  of  the  kiln  so  tem- 
pers the  latter  when  melted  in  saggars,  that  it  is  much 
more  difficult  to  grind  than  when  suddenly  quenched  in 
water  on  flowing  from  a  frit-kiln. 

The  various  materials  enumerated  in  the  preceding 
chapter  all  enter  into  glaze  formulas.  There  the  com- 
position and  properties  of  flint,  feldspar,  Cornish-stone, 
whiting,  and  the  clays  have  been  sufficiently  discussed  to 
throw  the  necessary  light  on  their  use  in  this  connection. 

The  various  lead  preparations  used  in  glaze  making, 
namely  white  and  red. lead,  and  litharge,  are  easily  ob- 
tainable in  the  market  in  a  high  state  of  purity.  The 
testing  of  these  is  also  not  difficult,  and  can  be  found  in 
books  on  technical  analysis. 

It  remains  merely  to  call  attention  to  a  superstition 
among  potters,  created  by  commercial  rivalry  among 
white-lead  manufacturers,  that  traces  of  acetates  in  white 


126  THE   CHEMISTRY   OF   POTTERY. 

lead  are  of  deleterious  influence  on  the  glaze.  A  real  dan- 
ger lies  in  the  mistaken  or  ignorant  use  of  lead  sulfate  in 
the  place  of  white  lead.  This  sulfate  is  now  produced  by 
direct  oxidation  of  galena  and  put  upon  the  market  under 
names  tending  to  confound  it  with  the  former  product. 

Borax  is  obtainable  of  sufficient  purity,  though  it  is 
usually  well  to  examine  it  for  the  possible  presence  of 
sodium  sulfate. 

Boracic  acid  may  contain  ammonium  sulfate  and  prove 
troublesome  by  introducing  sulfates  into  the  frit.  As 
much  as  10.28  per  cent.  SO8  has  been  found  by  the  wri- 
ter in  the  light-brown  flaky  variety. 

Soda  is  more  conveniently  employed  in  the  form  of 
soda-ash,  as  the  large  amount  of  water  of  crystallization 
in  sal-soda  is  troublesome  in  the  melting  of  a  frit  con- 
taining it,  and  furthermore  the  efflorescence  of  the  salt, 
when  standing  in  open  barrels  is  liable  to  cause  serious 
changes  in  the  composition  of  mixtures  made  with  it. 

It  is  important  to  examine  all  soda-ash  for  the  pres- 
ence of  sulfate.  That  made  by  the  Solvay  process,  and 
now  becoming  common  in  the  market,  is  free  from  such 
contamination  however. 

Potash,  where  its  introduction  as  potash-feldspar  is  not 
admissable,  because  of  the  amount  of  alumina  or  silica 
necessarily  accompanying  it,  is  used  as  pearl-ash,  or 
where  great  purity  is  required  as  bicarbonate  or  nitrate. 

On  account  of  its  variable  composition  pearl-ash  should 
always  be  subjected  to  chemical  analysis  previous  to  use 
and  because  of  its  hygroscopic  character,  its  moisture 
should  be  determined  and  allowed  for  whenever  it  is 
applied. 


CHAPTER  XI. 
MAJOLICA  AND  ENAMELED  TILE. 


ITH  exception  of  the  printing  and  hand  paint- 
ing in  vitrifiable  colors  and  gold,  and,  to  a 
more  limited  extent,  in  underglaze  colors,  on 
the  useful  articles  of  manufacture  coming  un- 
der the  head  of  the  kinds  of  ware  described  in 
the  last  chapter,  the  principal  commercial  development 
of  ornamental  pottery  in  the  United  States  has  been  in 
the  use  of  transparent  colored  glazes  on  modeled  surfaces. 
In  so-called  majolica,  of  which  many  articles  of  an  or- 
namental and  semi-useful  nature  are  made,  a  decorative 
effect  is  sought  to  be  attained  by  the  application  of  dif- 
ferently colored  glazes  to  different  parts  of  the  piece. 
The  colored  glazes  are  put  on  thin,  and  the  ware  is  gen- 
erally very  imperfect  technically,  the  glazes  being  mi- 
nutely crazed  and  the  body  very  porous  and  brittle. 

In  enameled  tile  the  individual  pieces,  modeled  as  a  rule 
in  bolder  relief  than  the  ornament  of  the  former,  are  colored 
in  monochrome,  through  a  very  heavily  glazed  surface. 
The  decorative  effect  being  attained  by  the  variation  in 
depth  of  the  glaze  through  the  approach  to  or  recess  of 
the  contour  of  the  design  from  the  surface  of  the  glass, 
lighter  and  darker  tints  of  the  color  appear  and  bring  out 
the  design  in  light  and  shade. 

To  a  certain  extent  also,  mottled  colors  in  imitation 


128  THE    CHEMISTRY   OF    POTTERY. 

of  marbles  are  used,  but  in  all  cases  the  decorative  effect 
sought  is  dependent  on  the  thickness  of  the  glaze,  which 
in  such  measure  refracts  the  light  and  makes  the  glaze 
brilliant. 

This  ware,  manufactured  and  used  very  extensively 
for  interior  decoration  in  kitchens,  halls,  bath  rooms, 
about  hearths  and  mantel  pieces,  is  artistically  as  well 
as  technically  much  better  than  that  previously  men- 
tioned ;  but  the  heavy  glazes  employed  to  make  it  dec- 
orative, offend  good  taste  as  well  as  make  it  practically 
impossible  in  manufacture  to  steer  the  product  uniformly 
perfect  through  the  "Scylla  and  Charybdis"  of  crazing 
and  shivering. 

The  body  and  glazes  of  both  of  these  kinds  of  ware  are 
very  much  the  same.  The  former  coincides  in  composi- 
tion with  the  body  of  cream-colored  wares,  but  is  as  a 
rule  not  so  hard,  because  the  manufacturers  of  ornamen- 
tal articles  are  more  careless  in  the  giving  of  a  thorough 
biscuit  fire,  than  those  who  are  faced  with  the  conditions 
imposed  on  useful  products, 

Tile,  which  must  burn  perfectly  level  and  true  in  size 
and  shape,  are  not  formed  from  the  mass  in  a  plastic 
state,  but  this  is  dried  and  reduced  to  a  powder  contain- 
ing from  eight  to  ten  per  cent,  moisture,  which  is  pressed 
into  shape  in  metal  dies  with  powerful  screw,  cam,  or 
hydraulic  presses. 

By  this  means,  bodies  are  obtained  that  are  little  liable 
to  warp  or  become  otherwise  untrue  in  drying  and  burn- 
ing, but  the  clay  particles  not  being  nearly  so  closely 
bonded  as  in  drying  down  from  a  plastic  condition,  the 


MAJOUCA   AND   ENAMELED   TILE.  1 29 

larger  pieces  are  with  many  clays  subject  to  cracking  or 
"  dunting"  as  it  is  called.  In  addition  to  the  other  con- 
ditions that  obtain  in  mixing  white  pottery  bodies,  this 
danger  must  be  kept  in  mind  in  selecting  a  body  suita- 
ble for  tile. 

The  clear  glazes  used  as  the  bases  of  the  colored  ones, 
for  these -industries,  are  both  raw  and  fritted.  The  for- 
mer resemble  closely  those  used  in  yellow  ware;  the  lat- 
ter must  be  more  fusible  and  brilliant  than  those  em- 
ployed for  white  granite  and  cream- colored  wares  and 
contain,  therefore,  more  lead  oxid  and  boric  anhydride 
and  rather  less  silica  and  alumina. 

However,  the  tints  imparted  by  the  chromogenic  oxlds 
to  glazes  are  so  profoundly  influenced  by  their  chemical 
composition  and  the  relative  proportions  of  their  basic 
and  acid  oxids,  that  the  widest  divergence  in  composi- 
tion must  be  looked  for. 

It  is  only  possible,  therefore,  to  give  for  illustration  a 
common  type  of  a  good  glaze  of  this  class,  as  the  follow- 
ing : 

0.25  Na.Cn  (2cSiO 

0.25  CaO    k2A1«0'loifx> 

0.50  Pbo  y         (°-5  ^u* 


i.o  RO 

It  may  be  prepared  by  first  melting  a  frit  of 
0.5  equivalent  borax,      95.5  parts. 
0.5  whiting,  25.0     " 

2.0          "  flint,         60.0     " 

its  formula  being 

0.5  Na2O  )  2.0  SiO2 
0.5  CaO    )i.oB2O3 


130  THE    CHEMISTRY   OF   POTTERY. 

and  its  combining  weight  124.5.     *n  order  to  make  the 
glaze  it  is  then  necessary  to  grind  together 

0.5  equivalent  of  the  frit,  62.25  parts. 

0.5          "  "         white  lead,  64.85     " 

0.2          "  "         china  clay,  25.9      " 

i.i  equivalents  "          flint,  33.0       " 

For  coloring  the  glazes,  cobalt,  nickel,'  copper, 
chrome,  manganese,  uranium,  and  iron  oxids  are  in  use. 

It  is  customary  to  grind  these  with  quartz  and  china- 
clay  previous  to  their  addition  to  the  glaze,  in  order  to 
insure  their  perfect  division,  that  they  may  dissolve  in 
th£  glazes  without  producing  spots  of  deeper  tint,  but 
produce  a  uniformly  colored  glass.  As  a  rule  there  is 
no  proportion  of  these  admixtures  bearing  any  rational 
relation  to  the  glazes  that  is  observed  ;  in  fact,  the  only 
thought  connected  .with  this  addition,  beside  that  of  divi- 
sion of  the  coloring  oxid,  is  cutting  down  its  tinctorial 
power,  so  that  those  of  higher  coloring  property  receive 
the  larger  additions.  As  a  result,  the  addition  of  these 
colors  changes  the  formula  of  the  glaze  to  a  greater  or 
less  degree.  It  would,  therefore,  be  a  more  rational 
proceeding  to  add  to  the  various  oxids,  in  grinding  the 
corresponding  colors,  the  quartz  and  china-clay  in  such 
proportion  as  to  give  compounds  of  the  same  acidity  as 
the  glaze  in  which  they  ar,e  to  be  used.  Any  desired 
proportion  could  then  be  added  to  the  glaze  without  ma- 
terially disturbing  the  relations  of  its  chemical  formula, 
which  is  of  some  moment  both  with  reference  to  the  fusi- 
bility and  the  coefficient  of  expansion.  Such  mix- 
tures, for  the  above  glaze,  would  be  composed  as  follows  : 


MAJOLICA   AND   ENAMELED   TILE.  131 

For  cobalt  color 

i.o  equivalent   cobalt  oxid  (Co3O4),  40.0  parts. 
0.2  "  china-clay,  25.9     " 

2.6  equivalents  flint,  78.0     " 

and  for  the  other  colors,  like  amounts  of  clay  and  flint, 
with  the  following  amounts,  respectively,  of  the  oxids  : 

37.4  nickel  oxid  (NiO). 
39.8  copper  oxid  (CuO). 
38.3  chromic  oxid  (Cr2O3). 
38.2  manganese  oxid  (Mn3O4). 
40.0  ferric  oxid  (Fe2O3). 
70.7  uranium  oxid  (U3O4). 

A  better  method  than  this,  insuring  the  mo'st  perfect 
division  of  the  coloring  oxid  and  no  disarrangement  of 
the  chemical  formula  of  the  glaze  by  its  addition,  is  to 
substitute  the  oxid  for  one-half  of  the  basic  equivalents 
of  the  glaze  formula  and  melt  the  resulting  mixture.  A 
series  of  colored  frits,  each  containing  one  of  the  color- 
ing oxids  in  definite  chemical  proportion  can  thus  be 
made,  which  may  be  added  in  any  desired  measure,  to 
the  clear  glaze,  in  order  to  produce  the  various  colors. 
The  formula  of  these  colored  frits  would  then  be,  for  the 
clear  glaze  given 


0.125  Na2O)  C 

0.125  CaO      o.2Al209]oc 
0.25    PbO    )  (°-,5. 


0.50    RO 

The  different  coloring  oxids  take  the  place  of  RO. 
Thus,  the  copper  frit  would  be  made  by  melting  together 
the  mixture  : 


132  THE   CHEMISTRY  OF    POTTERY. 

0.125  equivalent   powdered  borax,  23.9    parts. 
0.125  whiting,  6.25     " 

0.25  "  white  lead,  32.4       " 

0.5  copper  oxid,          19.9       " 

2.1      equivalents  flint,  63.0       " 

0.25    equivalent    boracic  acid,          15.5       " 
This  would  yield  135.7  parts  of  glass,  that  are  then 
ground  with  0.2  equivalent  of  china  clay,  25.9  parts.    In 
the  other  frits  the  copper  is  replaced  by  the  equivalent 
weights  of  the  respective  coloring  oxid. 

As  these  frits  are  used  in  small  amounts,  melting  them 
on  the  hearth  of  the  frit-kiln  would  not  be  practical.  The 
advantages  of  the  frit-kiln  on  a  small  scale  can  be  at- 
tained, however,  by  melting  in  large  crucibles,  pierced 
in  the  bottom  with  a  hole  three-eighths  of  an  inch  in  di- 
ameter. As  many  of  these  as  frits  are  desired  to  be 
melted  at  one  time,  are  placed  side  by  side  on  the  flat 
hearth  of  a  furnace  having  a  hole  two  and  one-half  inches 
in  diameter  under  each  crucible.  Into  each  of  these, 
which  must  be  easily  reached  and  filled  from  above 
through  corresponding  openings  in  the  crown  of  the  fur- 
nace, its  frit-mixture  is  introduced,  which  on  becoming 
sufficiently  fluid,  runs  from  the  hole  in  the  bottom,  falling 
through  the  opening  in  the  hearth,  into  a  suitable  recep- 
tacle filled  with  water  below. 

The  principal  raw  materials  of  these  glazes,  with  ex- 
ception of  the  coloring  oxids,  have  been  previously  dis- 
cussed. Cobalt  is  commonly  met  with  in  the  market  as 
black  oxid,  prepared  for  use  in  Saxony  and  Wales.  All 
commercial  preparations  contain  nickel  oxid  in  larger  or 
smaller  proportion,  as  the  impurity  affecting  the  tint  of 


MAJOLICA  AND   ENAMELKD   TII^.  133 

the  glass  in  which  the  oxids  are  used.  The  amount  of 
nickel  oxid  contained  in  the  Saxon  brands  R.  K.  O.  and 
F.  K.  O.  is  from  five  to  six  per  cent. ;  in  G.  K.  O.  there 
is  from  two  to  three  per  cent.,  and  in  F.  F.  K.  O.  one- 
half  per  cent. 

The  Welsh  oxid,  seen  in  this  market,  generally  con- 
tains about  five  per  cent,  nickel  oxid.  Nickel  oxid  itself 
is  not  used  very  extensively  from  the  fact  that  in  the  more 
convenient  glazes,  which  are  rich  in  lead  oxid  and  bo- 
racic  acid,  it  is  very  liable  to  cause  turbid  loam-colored 
separations,  and  to  shade  badly  from  brownish  to  green- 
ish tints.  It  is,  however,  serviceable  in  the  more  alka- 
line glazes  and  those  poor  in  boracic  acid. 

Nickel  oxid  is  obtainable  quite  pure  of  domestic  man- 
ufacture. 

Copper  oxid  in  the  form  of  copper  scale  or  black  oxid, 
is  of  such  varying  origin  as  found  in  the  market,  that  it 
is  advisable  to  always  subject  it  to  examination  previous 
to  use.  As  only  glasses  containing  lead  oxide  in  at  least 
some  proportion  are  in  use,  and  the  glost-fire  is  of  neces- 
sity oxidizing  only  the  green  and  blue-green  tints  of  the 
oxid  are  known  in  our  domestic  pottery.  The  sub-oxid 
supplied  to  glass  makers  for  ruby  glass,  finds  no  appli- 
cation. 

Chromic  oxid  is  very  commonly  made  by  the  potters 
themselves,  by  roasting  an  intimate  mixture  of  potassium 
bichromate  and  sulfur  in  the  top-kiln  or  flue  of  the  glost- 
oven,  and  grinding  and  washing  the  product  until  free 
from  alkaline  sulfid.  Used  in  larger  proportion,  chromic 
oxid  produces  intransparent  green  enamels,  and  even  in 
smaller  amounts  it  is  liable  to  separate  from  the  clear 


134  THE   CHEMISTRY   OF   POTTERY. 

greenish-yellow  glass  in  deep  green  intransparent  flecks. 
It  is  frequently  the  custom,  therefore,  because  of  the  bet- 
ter division  of  the  oxid,  to  introduce  it  in  the  form  of 
mercuric  chromate  or  lead  chromate.  The  latter  should 
never  be  used  without  previous  examination,  as  it  is 
manufactured  mainly  as  a  pigment,  and  as  such  is  fre- 
quently adulterated  to  render  it  lighter  in  tint. 

Manganese  is  an  important  chromogen  for  pottery  gla- 
zes and  is  used  in  several  forms.  Where  brown  tints  are 
desired,  the  presence  of  iron  oxid  is  not  only  not  objec- 
tionable, but  is  needed,  the  amount  depending  upon  the 
required  tint.  The  most  common  form  in  which  man- 
ganese finds  application  is  as  black  oxid  or  pyrolusite. 
It  is  sold  in  a  variety  of  grades,  varying  considerably  in 
the  amount  of  contained  sandy  matter  and  ferric  oxid,  so 
that  where  exact  and  economical  work  is  contemplated, 
it  should  always  be  analyzed  before  use.  Where  yellow- 
ish-brown tints  are  required  Turkey  umber  is  often  em- 
ployed, though  its  use  is  neither  economical  nor  rational, 
inasmuch  as  the  color  imparted  by  this  substance  is  due 
to  the  contained  manganous  and  ferrous  oxids.  It  would 
be  better  and  cheaper  to  add  these  substances  in  approx- 
imately pure  form,  as  commercial  umber  varies  consid- 
erably in  the  percentage  of  these  present.  Nor  is  it 
graded,  commercially,  so  that  one  could  form  a  rough 
idea  of  the  proportion  of  its  important  constituents ;  for 
being  mainly  used  as  a  pigment,  physical  rather  than 
chemical  properties  determine  its  grade. 

Umber  usually  contains  from  sixteen  to  twenty  per 
cent,  of  manganous  oxid  and  from  thirty  to  thirty-eight 
per  cent,  ferric  oxid.  It  generally  contains  also  quite 


MAJOLICA  AND   ENAMELED   TILE.  135 

an  appreciable  amount  of  gypsum,  which  is  objectionable 
on  account  of  the  introduction  of  sulfates  into  the  glaze. 

Where  the  reddish  and  wine-colored  tints  of  manga- 
nese are  desired,  it  is  essential  to  use  preparations  en- 
tirely free  from  all  traces  of  iron.  In  this  case  it  is  com- 
mon to  use  manganous  carbonate.  It  is  important,  how- 
ever, not  to  trust  commercial  preparations,  which  often 
contain  as  much  iron  as  common  pyrolusite.  Not  infre- 
quently the  commercial  carbonate  contains  considerable 
amounts  of  calcium  carbonate,  not  intentionally  added, 
but  derived  probably  from  a  recovered  manganese. 

The  following  analyses  are  of  preparations  of  this  char- 
acter : 

Per  cent.  Per  cent. 

Sandy  matter o.io  ^Z-1! 

Alumina o.  1 1 

Manganous  carbonate 52.15  63.17 

Calcium  carbonate 47.07  21.55 

Magnesium  carbonate 0.57  2.11 

100.00  100.00 

As  pure  manganous  carbonate  is  by  no  means  a  stable 
product,  but  loses  moisture  and  carbonic  acid  on  expos- 
ure to  the  air,  it  should  be  assayed  from  time  to  time,  or 
it  should  be  converted,  by  heating  in  the  presence  of  air, 
into  mangano-manganic  oxid  and  weighed  as  such. 

Uranium  is  offered  in  the  market  as  oxid,  ranging  in 
color  from  a  bright  canary  to  an  orange-yellow.  The 
preparation  is  really  a  hydrate,  and  a  difference  of  five 
per  cent,  in  the  amount  of  loss  on  glowing  different 
specimens  under  similar  conditions  has  been  found  by 
the  writer.  The  colors  obtained  with  uranium  vary 
with  the  composition  of  the  glaze  from  greenish  lemon- 


136  THE   CHEMISTRY   OF    POTTERY. 

yellow  to  orange.  The  black  color  imparted  to  certain 
fluxes,  used  in  the  verifiable  colors  of  overglaze  paint- 
ing, is  not  producible  in  the  glazes  and  under  the  firing 
conditions  of  pottery.  Pitch-blende,  used  by  European 
potters,  is  not  known  to  those  of  the  United  States. 

Ferric  oxid  is  a  useful  chromogen,  giving  orange  and 
brownish-yellow  tints.  It  is  readily  obtained  of  suffi- 
cient purity,  but  being  mainly  prepared  for  the  market 
as  a  pigment  or  polishing  powder,  physical  properties 
determine  its  commercial  grading  and  the  higher  priced 
samples  are  often  the  poorest  for  the  potter's  use.  Fer- 
ric oxid  should  never  be  used,  therefore,  without  pre- 
vious chemical  examination. 

Specimens  prepared  directly  from  hematite  and  other 
iron  ores  generally  contain  more  or  less  sandy  matter  ; 
those  made  by  roasting  copperas  often  contain  injurious 
residues  of  sulfuric  acid. 

The  crocus  powders  to  which  potters  are  particularly 
partial,  as  most  English  pottery  receipts  call  for  "  cro- 
cus martis"  in  place  of  ferric  oxid,  are  treacherous  be- 
cause of  the  foreign  additions  they  almost  invariably 
contain,  which  have  been  added  to  brighten  the  color  or 
improve  their  character  as  polishing  agents.  A  ' '  crocus 
martis' '  sold  to  a  pottery  was  found  by  the  writer  to  con- 
tain : 

Per  cent. 

Ferric  oxid 47-14 

Alumina 2.31 

Silica 13-65 

Barium  sulfate 37-4* 

100.51 


MAJOLICA   AND    KNAMKLED   TILK.  137 

Underglaze  crimson  is  used  instead  of  copper  and  gold 
for  pink  and  red  glazes,  the  colors  being  much  more 
easily  obtained,  though  far  inferior  in  quality  to  those 
producible  with  the  latter  agents.  The  color  consists 
mainly  of  stannic  oxid,  containing  a  trace  of  chrome  in 
minute  division.  The  analysis  of  a  good  quality  of  the 
color  of  English  manufacture  ran  as  follows  : 

Per  cent. 

Silica 10.02 

Alumina 1.57 

Chromic  oxid strong  trace 

Lime 20.32 

Carbon  dioxid 1 .00 

Stannic  oxid  (by  difference) 67.09 


100.00 


Colors  with  increased  lime  and  a  reduction  of  silica 
impart  a  disagreeable  purplish  cast  to  the  glaze.  The 
composition  of  the  latter  also  affects  the  tint  considera- 
bly ;  boracic  acid,  particularly,  can  only  be  used  in 
small  proportion. 

Stannic  oxid  is  used  to  convert  the  clear  glazes  into 
white  intransparent  enamels.  As  its  effect  depends  upon 
its  being  insoluble  in  the  glass,  and  clouding  it  by  re- 
maining suspended  in  it,  its  physical  character  is  all  im- 
portant. Many  commercial  preparations  of  the  oxid  are 
much  too  dense,  collecting  in  little  concretions  in  the 
glass  and  giving  it  a  curdy  appearance  without  much 
covering  power. 

To  insure  a  perfect  division  of  the  tin  oxid  in  the 
glaze  it  is  best  to  introduce  it  in  the  form  of  a  ' '  putty- 


138  THE   CHEMISTRY  OF   POTTERY. 

powder,"  with  the  lead  oxid.  This  is  done  by  first  pre- 
paring an  alloy  of  the  metallic  lead  and  tin  and  carefully 
oxidizing  it  in  a  flat  pan  or  on  the  open  hearth  of  a  fur- 
nace, with  plentiful  excess  of  air,  raking  off  the  oxid 
from  the  surface  of  the  molten  metal  as  it  forms. 

The  oxid  is  then  ground  in  water  for  a  short  time  in 
a  tumbler  mill  and  floated  from  the  flattened  grains  of 
unoxidized  metal. 

The  dried  powder  must  be  assayed  and  its  addition  to 
the  glaze  regulated  by  its  proportions  of  the  respective 
oxids. 


CHAPTER  XII. 
WHITE  ENAMELED  BRICK. 


HE  development  of  fire-proof  building  has 
made   a   permanently   white  covering  for 
walls,  imperative  as  a  substitute  for   lath 
and  plaster.    The  light-conditions  of  our 
large  buildings  in  crowded  city  quarters, 
demand  that  this  covering  be  highly  reflecting,  easily 
cleaned,  and  as  permanent  as  the  structure  itself. 

Answering  these  and  many  minor  conditions,  brick 
with  one  or  two  faces  covered  with  a  white  glazed  or 
enameled  surface  are  being  supplied  to  meet  the  demand 
for  such  walls  for  ware-cellars  and  subways,  the  halls 
and  corridors  of  public  buildings  and  railway  depots, 
the  operating  rooms  of  hospitals  and  light-shafts  and 
light  walls  of  the  narrow  courts  of  office  buildings. 

The  technical  condition  of  the  manufacture  of  these 
brick,  namely  the  melting  of  a  solid  enameled  surface  on  a 
bulky  ware  of  coarse  clay,  together  with  the  cost  of  fre- 
quent and  careful  handling, has  presented  such  difficulties 
that  the  manufacture  has  not  kept  pace  with  the  demand ; 
hence  a  large  portion  of  the  supply  comes  to  us  from 
abroad. 

The  methods  of  making  these  brick  are  two.  The 
one  consists  in  covering  the  surface  to  be  exposed,  with 
a  tin  enamel  of  sufficient  covering  power  to  present  a 


140  THE    CHEMISTRY   OF   POTTERY. 

smooth  white  surface  completely  concealing  the  clay  un- 
derneath; the  other,  in  coating  the  face  to  be  exposed, 
with  an  engobe  or  "slip"  of  china-clay  or  of  a  body  re- 
sembling that  of  white  ware  and  melting  a  transparent 
glaze  over  this. 

Where  the  character  of  the  clay  is  such  that  it  can  be 
burned  at  the  same  heat  as  the  glaze  or  enamel  it  will 
bear,  the  aim  is  to  finish  the  piece  with  its  coating  in  the 
clay  state  and  subject  the  product  to  but  one  fire. 

Tin  enameled  brick  are  best  made  on  the  plan  of  the 
white  stove  tile  used  on  the  continent  of  Europe.  For 
these,  clays  rich  in  the  carbonates  of  lime  and  magnesia 
are  best  suited.  The  enamel  will  lie  on  them  as  a  smooth 
and  uniform  glass  without  danger  of  beading  up  and 
leaving  portions  of  the  surface  uncovered,  and  if  the  car- 
bonates be  present  in  sufficient  amount,  without  crazing. 
Such  clays  should  bake  to  the  hardness  of  good  building 
brick  at  silver-melting  heat,  which  is  the  best  tempera- 
ture for  flowing  the  lead-tin  enamels. 

As  a  type  of  such  clays,  burning  to  a  light  buff  color 
and  of  sufficient  hardness  at  the  melting-point  of  silver 
(960°  C.),  one  from  Hamilton  county,  Ohio,  may  be 
taken,  which  gave  the  following  analysis : 


Silica  

The  entire 
clay 
Per  cent. 

41  66 

The  portion  insol- 
uble in  H2SO4 
and  Na2CO3 
Per  cent. 

25.50    . 
2.03 
0.00 

0.17 
0.15 

O.^l 

•93 

T  ^   77 

6  07 

Potash  .  . 

2.  it; 

WHITE   ENAMELED   BRICK.  141 

The  portion  insol- 

The  entire  uble  in  H2SO4 

clay  and  Na3CO8 

Per  cent.  Per  cent. 

Soda 0.50  0.31 

Carbon  dioxid 13.24  o.oo 

Combined  water 3.88  o.oo 


99.40  28.69 

This  would  bear  an  enamel  of  German  manufacture, 
very  white,  of  excellent  covering  power  and  entirely  frit- 
ted, that  analyzed  as  follows  : 

Per  cent. 

Lead  oxid 28.56 

Stannic  oxid 9.4° 

Alumina 4.79 

Ferric  oxid 0.08 

Lime * 0.60 

Potash 8.82 

Loss  on  ignition 0.93 

Boracic  acid none 

Silica  (by  difference) 46.82 

100.00 
The  chemical  formula  of  the  glass  then  is: 

0.551 1  PbO   ) 

0.4029  K2O    Y  0.2035  A12O3  3.358  SiO2 

0.0460  CaO    ) 


i.o     RO 
with  9.4  per  cent,  suspended  SnO2. 

But  tin-enameled  brick  made  with  calcareous  clays 
and  finished  in  one  fire  are  not  yet  common  in  the  United 
States.  In  most  cases  the  enamel  is  applied  to  ordinary 
buff  and  even  red  brick  and  flowed  in  a  second  heat. 


142  THE    CHEMISTRY   OF    POTTERY. 

A  brick  of  American  manufacture  was  found  by  the 
writer,  which  did  not  quite  conform  to  either  of  the 
above.  The  enamel  had  been  applied  to  the  "biscuit" 
or  once  baked  body,  but  between  the  brick  proper  and 
the  enamel  was  an  engobe,  apparently  applied  for  the 
twofold  purpose  of  helping  to  conceal  the  body,  as  the 
enamel  in  itself  seemed  of  insufficient  covering  power  and 
for  supplying  a  surface  to  which  the  enamel  would  ad- 
here without  beading  and  crawling,  as  the  body  of  the 
brick  contained  insufficient  lime  to  insure  this. 

The  material  of  the  brick  consisted  of 

Per  cent. 

Silica 67.99 

Alumina •  • 24.97 

Ferric  oxid 1.12 

Lime 3.63 

Magnesia 1.46 

Alkalies    1.24 

100.41 

The  enamel  carefully  dressed  from  the  surface  gave  the 
analysis : 

Per  cent. 

Stannic  oxid 1 1 .42 

Lead  oxid 33. 1 1 

Alumina  7.86 

Lime 4.53 

Magnesia 0.36 

Alkalies    4.87 

Boracic  acid none 

Silica  (by  difference) 38.85 

100.00 


WHITE   ENAMELED    BRICK.  143 

the  chemical  formula  of  the  glass  then  being : 
o.i798K2O  ^ 

0.3037  CaO  V  0.2654  A19O9  2.252810, 
0.5165  PbO  J 


i.o     RO 

with  11.42  per  cent,  suspended  SnO2,  though  so  poorly 
divided  that  the  enamel  looked  curdy  and  was,  as  already 
stated,  of  poor  covering  power. 

The  alumina,  as  found  by  the  analysis,  is  unquestion- 
ably higher  than  was  originally  introduced  in  the  glaze  ; 
but  in  fusion  some  would  be  dissolved  and  taken  up  from 
the  surface  of  the  body  or  engobe  beneath.  Taken  as 
the  analysis  shows  it,  the  enamel  would  be  produced  by 
the  following  formula : 

0.5165  equivalent  white  lead 66.98  parts. 

°-3°37          "  whiting 15.20     " 

0.1798          "  feldspar 50.08     " 

0.0856          •'  china  clay 11.10     " 

T.oooo          "  flint 30.00     " 

Tin  oxid 19.86     " 

The  engobe  was  so  eaten  into  by  the  glaze  that  it  was 
difficult  to  chip  off  a  pure  specimen.  The  best  that  could 
be  obtained  was  of  the  following  composition : 

Per  cent. 

Silica 44.56 

Stannic  oxid 2.75 

Lead  oxid : 25.22 

Alumina 19.50 

Lime 0.57 

Magnesia 2.10 

Alkalies 5.30 

100.00 


144  THE   CHEMISTRY   OF   POTTERY. 

Taking  the  stannic  oxid,  in  the  above,  as  indicative 
of  the  amount  of  enamel  present  in  the  specimen  analy- 
zed and  deducting  the  proportionate  amounts  of  the  other 
constituents,  the  engobe  proper  must  have  had  (in  the 
burned  condition)  the  percentage  composition  : 

Per  cent. 

Silica 46.51 

Lead  oxid 22.79 

Alumina 23-27 

Lime  and  magnesia 1.97 

Alkalies 5.46 

99-99 
which  wo.uld  approximately  be  given  by  the  mixture  : 

Feldspar 50  parts. 

White  lead 25      " 

China-clay 33      " 

The  other  type  of  "  enameled  "  brick  mentioned,  hav- 
ing a  clear  glaze  over  a  white  engobe  which  covers  and 
conceals  the  buff-body  of  the  brick,  is  made  from  a  sandy 
fire-clay  and  finished  with  its  two  coatings  in  the  clay 
state,  being  subjected  when  dry  to  but  one  fire.  The 
temperature  of  this  is  quite  high,  sufficient  to  melt  the 
pyrometric  cones  nine  or  ten,  the  glazes  used  not  being 
plumbiferous,  but  of  the  porcelain  type. 

The  body  of  an  English  brick  of  this  character  had, 
in  its  burned  condition,  the  composition: 

Per  cent. 

Silica 72.36 

Alumina 24.97 

Ferric  oxid 0.74 

Lime i  .34 


WHITE   ENAMELED   BRICK.  145 

Per  cent. 

Magnesia 0.30 

Alkalies i.oi 


100.72 

The  engobe  was  a  pure  china-clay.      The  glaze  was 
composed  of 

Per  cent. 
Silica  (by  difference)  .......................     66.67 

Alumina  ...................................     20.64 

Lime  ......................................       7.68 

Magnesia  ...................................       0.33 

Potash  .....................................       4.68 

IOO.OO 

its  chemical  formula  being 


0^     5-76Si08. 

In  the  severe  fire,  to  which  the  ware  has  been  sub- 
jected, the  glaze  has,  of  course,  taken  up  alumina  and 
silica  from  the  engobe,  so  that  its  original  composition 
would  not  accord,  in  the  proportion  of  these  elements, 
with  the  formula  derived  from  the  analysis  of  the  fired 
product,  as  above  given.  A  comparison  of  this  formula 
with  those  of  the  stoneware  slips,  shows  by  how  much 
approximately  the  proportions  of  alumina  and  silica  are 
perhaps  too  high. 

The  formula  as  it  stands  would  be  produced  by  the 
following  mixture  : 

0.26  equivalent  feldspar  ..............  72.41  parts. 

0.74          "  whiting  ...............  37.00      " 

0.78  china-clay  ............  101.01      " 

2.64  eqi  valeuts  flint  ..................  79.02      '  ' 


146  THE   CHEMISTRY   OF   POTTERY. 

Trials  made  on  the  basis  of  the  above  formula  with 
systematic  reduction  of  the  china-clay  and  if  need  be  of 
the  flint,  would  in  a  very  few  trials  give  the  composition 
of  a  glaze  such  as  a  particular  clay  and  the  fire  required 
to  bake  it  would  need. 

The  clear  glaze  chipped  from  a  brick  of  similar  char- 
acter of  American  manufacture  gave  the  analysis  : 

Per  cent. 

Silica  (by  difference)  .......................  65.67 

Alumina  .................................  .  .  20.20 

Ferric  oxid  .................................  0.49 

Lime  ......................................  6.27 

Magnesia  ...................................  0.88 

Alkalies1  ...................................  6.49 

IOO.OO 

the  chemical  formula  then  being 


o.6456CaO  j 

i.o       RO 

The  adaptation  of  a  clay  and  enamel  or  of  a  clay,  en- 
gobe,  and  glaze  to  each  other,  in  both  of  these  kinds  of 
brick,  involves  considerable  empiric  experiment,  par- 
ticularly if  it  is  aimed  to  finish  them  in  one  fire.  Not 
only  do  the  difficulties  due  to  differences  of  their  coeffi- 
cients of  expansion  arise,  causing  crazing  or  shivering  of 
the  glaze  or  ehamel  which  must  be  avoided,  but  also  the 
shrinkage  conditions  in  the  clay  state  must  be  so  met 
that  the  surface  coverings  do  not  shell  off  in  the  drying 
or  loosen  so  as  to  turn  up  in  the  fire,  melting  back  in 


1  Combining  weight,  46.3. 


WHITE    ENAMELED    BRICK.  147 

beads  with  exposure  of  uncovered  patches  of  the  brick 
surface. 

A  consideration  that  has  received  very  little  attention 
thus  far,  but  is  of  considerable  moment,  is  that  the 
glaze  or  enamel  itself  must  resist  all  action  of  the  atmos- 
phere and  the  body  of  the  brick  should  be  of  such  den- 
sity that  the  combined  action  of  moisture  and  frost  does 
not  cause  the  white  coating  to  split  off.  The  majority 
of  the  brick  now  placed,  it  is  true,  are  not  subjected  by 
their  position  to  the  latter  danger,  but  it  is  not  unlikely 
that  in  time  an  important  use  will  be  the  facing  of  the 
walls  of  light  courts,  which  are  not  under  roof. 

The  glazed  surface  cannot,  in  itself,  be  depended 
upon  for  preventing  the  rain-water  from  penetrating  the 
brick,  as  it  would  find  ample  access  to  the  body  of  the 
same  through  the  mortar-joints.  From  such  brick, 
saturated  with  water  and  subjected  to  several  frosts, 
the  impervious  surface  not  allowing  the  expansion  of  the 
ice  through  absence  of  pores,  is  very  soon  pushed  off. 
Hence,  glazed  or  enameled  brick,  in  order  to  be  of  prime 
quality,  should  be  very  nearly  vitreous  in  body.  It  is, 
of  course,  out  of  the  question  to  attain  this  with  the  soft- 
fired  calcareous  body  of  the  first  type  and  these  brick 
should  only  be  used  for  interior  work.  A  specimen  of 
the  kind  described,  having  a  feldspar-lead  china-clay 
engobe  between  the  body  and  the  tin  enamel,  absorbed 
eighteen  per  cent,  of  water.  It  showed  cracking  and 
loosening  of  its  enamel  when  saturated  with  water  and 
frozen  and  thawed  twice.  In  five  freezings  and  thaw- 
ings  it  was  completely  ruined. 


148  THE   CHEMISTRY   OF   POTTERY. 

Brick  of  the  second  type  with  even  a  porosity  allowing 
an  absorption  of  five  per  cent,  of  their  weight  of  water, 
withstood  thirty  freezings  and  thawings,  without  dam- 
age to  the  coated  surface.  There  are  few  brick  of  even 
this  type  in  the  market,  whether  of  foreign  or  domestic 
manufacture,  as  dense  as  this.  Most  of  them  will  take 
up  ten  per  cent,  by  weight  of  water,  which  is  too  much 
for  safe  out  of  door  exposure. 

The  hard  alkali-alkaline  earth  glazes  are  far  the  best 
for  resisting  atmospheric  influences,  but  even  lead  glazes 
and  enamels,  if  of  suitable  composition,  may  be  relied 
on  for  retaining  a  bright  reflecting  surface. 

In  order  to  test  if  such  glazes  are  in  no  danger  of  suf- 
fering surface  decomposition,  in  the  course  of  time,  by 
the  action  of  moisture  and  carbon  dioxid,  the  most  rapid 
and  practical  method  is  that  of  Professor  Rudolf  Weber. 
The  glazed  surface  is  exposed  for  twenty-four  hours 
under  a  bell  jar  to  the  fumes  of  highly  concentrated 
hydrochloric  acid.  Dried  in  an  atmosphere  free  from 
dust,  the  surface  must  remain  perfectly  bright  or  show 
at  most  the  very  faintest  clouding.  Marked  clouding 
indicates  a  decomposition  of  the  surface,  which  may  be 
further  seen  ori  wiping  off  the  cloud  of  salts  and  separa- 
ted silica  in  a  decided  iridescence  of  the  glass. 


CHAPTER  XIII. 
FLOOR-TILE  AND  TERRA-COTTA. 


HE  raw  materials  of  the  unglazed  wares  for 
architectural  use  and  ornament  are  almost  co- 
extensive with  the  argillaceous  minerals  of 
the  earth.     Their  final  selection  for  the  sev- 
eral purposes  depends  upon  the  conditions  of 
use  and  upon  the  necessary  or  accidental  con- 
ditions of  manufacture. 
The  first  considerations  are  with  reference  to  physical 
properties,  allowing  facile  shaping  and  the  faultless  dry- 
ing and  burning  of  the  ware. 

In  plastic-formed  objects,  particularly  in  very  large 
pieces  of  architectural  terra-cotta,  the  degree  of  plas- 
ticity and  the  binding  property  of  the  clay  are  mat- 
ters of  very  great  moment,  which  in  the  present  state  of 
knowledge  can  only  be  safely  determined  empirically  and 
on  a  scale  of  actual  work. 

The  plasticity  of  the  clay  must  be  ample  to  allow  for 
the  easy  forming  of  the  mass  in  molds,  or  its  modeling 
free-hand,  and  also  allow  heavy  masses  in  bold  relief  to 
sustain  their  own  weight.  Yet  the  clay  dare  not  be  so 
plastic  that  it  be  strained  in  the  shaping  and  twist  and  be 
contorted  in  drying  and  in  the  fire.  To  a  certain  degree 
the  evils  of  an  over-plastic  and  a  very  fine  grained  ma- 
terial are  overcome  by  adding  once  fired  clay  (''grog") 
in  coarse  grains  to  the  mass. 


150  THE   CHEMISTRY   OF    POTTERY. 

Such  material  must  have  sufficient  binding  property  to 
resist,  without  cracking,  the  strain  of  a  shrinking  sur- 
face on  a  less  shrunken  interior,  during  the  operations  of 
drying  and  burning, — both  attended  by  appreciable  re- 
ductions of  mass  from  the  surface  inward. 

Dust-pressed  ware,  such  as  flooring  tile  and  molding 
brick  for  belt-courses,  require  a  material  that  does  not 
form  center-cracks,  from  the  surface-sealing  of  the  clay 
during  pressing,  before  the  air  contained  in  the  powder 
has  escaped,  a  phenomenon  known  as  "busting."  The 
material  must  further  allow  the  ware  to  dry  and  be 
burned  without  warping,  cracking  through  ("dunting") 
or  becoming  covered  with  surface-cracks  ("checking"). 

The  selection  of  a  proper  kiln-temperature  for  burning 
the  ware  is  a  difficult  matter  that  can  also  only  be  done 
empirically.  As  a  rule  in  these  manufactures  a  variety 
of  colors  are  required  and  the  problem  of  temperature  is 
the  finding  of  the  heat  that  will  bake  to  proper  hardness 
the  largest  number  of  the  available  clays,  so  that  by  mix- 
ing and  suitable  disposition  in  the  kilns,  as  great  a  variety 
of  colors  as  possible  may  be  burned  in  one  and  the  same 
fire. 

The  temperatures  employed  in  these  industries  vary 
from  that  of  the  melting-point  of  silver  to  that  of  pyro- 
metric  cone  ten.  As  a  rule,  too,  they  have  been  derived 
haphazard  from  the  heat  found  best  for  one  clay  and  one 
particular  kind  of  ware  and  not  from  systematic  trial  of 
all  the  clays  that  are  likely  to  enter  into  the  manufacture. 
As  a  result  many  establishments  labor  under  the  serious 
difficulty  that  a  part  of  their  products  are  far  less  perfect 


FI^OOR-TII/E   AND   TKRRA-COTTA.  151 

than  others,  because  the  whole  are  burned  in  the  same 
fire,  which  is  only  suited  to  ware  from  one'or  two  of  the 
clays.  Or,  again,  in  order  to  meet  this  difficulty,  the 
colors  which  can  not  be  properly  produced  from  the  local 
clays  at  the  established  temperature,  must  be  gotten 
from  clays  obtained  from  a  distance,  thus  burdening  the 
manufacture  with  great  expense. 

The  conditions  thus  imposed  by  chance  are  in  the  ma- 
jority of  cases  accepted  as  inevitable  to  the  manufacture 
and  no  systematic  effort  is  made  to  readjust  and  correct 
them. 

While  it  is  true  that  mere  analyses  of  materials  are  of 
little  or  no  avail  in  the  solving  of  such  problems,  the 
training  of  -the  technical  chemist  fits  him  better  to  carry 
out  systematically  the  many  empiric  trials  necessary  for 
the  readjustment  of  the  conditions  or  their  proper  estab- 
lishment originally  in  these  works.  The  opportunity 
afforded  workers  of  such  training,  to  study  the  problems 
of  these  arts  in  actual  manufacture,  will  alone  lead  to 
the  finding  of  physical  data,  obtainable  in  the  labora- 
tory, by  which  the  behavior  of  the  various  clays  can  be 
foretold  and  bases  of  rational  manufacture  worked  out 
before  the  practical  establishment  of  plants. 

All  wares  for  architectural  use  must  adhere  closely  to 
definite  sizes.  The  shrinkage  of  the  clays  employed 
must  therefore  be  definitely  known  for  the  temperatures 
to  which  they  are  to  be  subjected,  so  that  the  molds  and 
dies  can  be  made  of  the  proper  dimensions.  Frequently 
pieces  of  the  same  pattern,  but  of  different  colors,  alter- 
nate in  a  frieze  or  floor.  It  is  therefore  a  great  conveni- 


152  THE    CHEMISTRY   OF   POTTERY. 

ence  if  the  shrinkage  of  all  the  adopted  colors  be  adjusted 
to  the  same  scale,  as  it  would  save  carrying  molds  and  dies 
of  the  same  design  in  a  variety  of  sizes.  The  question, 
however,  is  entirely  one  of  convenience  and  relative  ex- 
pense. In  the  case  of  dust-made  brick  with  plain  and 
simple  molding  faces  and  geometric  floor  tile,  it  is  per- 
haps best  that  the  bodies  employed  have  different  shrink- 
ages ;  as  in  that  case  the  dies,  after  wearing  too  large 
for  the  material  of  least  shrinkage,  can  be  chopped  out  to 
the  size  required  for  the  clay  of  next  greater  shrinkage, 
and  so  continue  until  they  have  done  service  for  the  entire 
color  scale.  Were  all  the  materials  on  the  same  scale  of 
shrinkage,  the  life  of  a  die  would  be  comparatively 
short,  as  it  could  no  longer  be  used  when  it  became  too 
large,  by  wear,  for  the  standard  size. 

The  question  of  shrinkage  is  most  aggravated  in  the 
case  of  encaustic  tile,  whether  made  plastic  or  of  dust- 
clay.  In  these,  the  colored  clays  forming  the  design  are 
inlaid  in  the  clay  making  the  body  of  the  ware.  As  the 
product  must  be  absolutely  level,  the  least  difference  in 
the  shrinkage  of  the  several  clays,  both  in  the  drying 
and  burning,  would  unfailingly  warp  the  tile,  either 
causing  them  to  "  buckle"  or  "  dish."  In  this  manu- 
facture it  is  also  absolutely  necessary  that  each  colored 
clay  require  the  same  heat  and  attain  an  equal  hardness 
with  the  others,  for  it  must  be  possible  to  inlay  any 
combination  of  the  color  scale  into  the  same  piece,  and 
therefore  occupy  the  identical  position  in  the  kiln.  Ad- 
vantage, therefore,  cannot  be  taken  of  the  greater  or 
less  difference  in  temperature  that  always  exists  between 


FI/)OR-TIIvK   AND   TERRA-COTTA.  153 

certain  parts  of  a  kiln,  for  burning  particular  colors,  as 
is  always  done,  where  individual  pieces  are  made  of  but 
one  mass. 

Soluble  salts  of  the  alkalies,  alkaline  earths,  or  of 
iron,  whether  pre-existent  in  the  clay  or  formed  while  it 
lies  as  a  damp  mass  or  in  the  fire,  are  dangerous  in  a 
variety  of  ways ;  chemical  examination  for  their  pres- 
ence or  the  likelihood  of  their  formation,  is  an  important 
though  greatly  neglected  step  in  the  selection  of  clays, 
for  ware  which  is  not  to  be  glazed. 

Gypsum,  if  present,  and  ferrous  sulfate  often  formed 
in  the  damp  clay  by  oxidation  of  the  finely  divided  iron 
pyrites  it  may  contain,  are  on  the  drying  of  the  finished 
pieces  brought  to  the  surface,  particularly  on  prominent 
points  and  ridges,  where,  on  account  of  the  readier 
evaporation,  larger  quantities  of  the  contained  moisture 
with  its  dissolved  salts,  are  drawn  by  capillarity.  Thus, 
on  the  eye-brows,  tip  of  the  nose,  lips,  and  chin  of  a 
terra-cotta  head  would  most  of  the  salts  be  lodged.  On 
burning,  these  surfaces,  which  in  the  clay  state  may 
have  shown  no  defect,  become  bodily  discolored  ;  red 
and  similar  colors  will,  through  the  presence  of  lime 
salts,  become  buff,  and  where  there  are  larger  amounts 
may  be  even  coated  with  a  whitish,  irremovable  scum  ; 
while  light-colored  clays  would,  through  the  presence  of 
the  iron  salts,  stain  brown  in  these  parts. 

Where  the  use  of  such  clays  cannot  be  avoided,  two 
methods  of  treatment  are  employed  to  escape  these  re- 
sults. One  consists  in  adding  to  the  clay  barium  chlorid 
and  carbonate,  in  order  to  render  the  salts  insoluble  by 


154  THB   CHEMISTRY   OF   POTTERY. 

double  decomposition,  preventing  their  carrying  to  the 
surface  with  the  movement  of  the  water  in  drying. 
Bighty-five  to  ninety  per  cent,  of  the  barium  necessary, 
as  shown  by  the  determination  of  sulfuric  acid  in  the 
clay,  is  added  in  the  form  of  chlorid,  dissolved  in  water, 
and  the  remainder  with  a  small  additional  excess,  in  the 
form  of  carbonate.  The  other  method  consists  in  pre- 
venting evaporation  from  the  right  side  of  the  ware,  by 
coating  it,  after  it  has  been  formed  and  finished,  but  be- 
fore drying,  with  crude  petroleum  or  tar.  As  the  evap- 
oration is  forced  to  take  place  entirely  from  the  uncoated 
surfaces,  the  salts  are,  in  the  drying  of  the  ware,  en- 
tirely carried  away  from  the  surface  and  lodged  where, 
in  the  setting  of  the  piece  in  the  wall,  they  are  unobjec- 
tionable. The  coating  material  of  the  face  must  be  of 
such  a  nature  that  it  burns  away  without  leaving  a  blem- 
ish. In  the  case  of  ware  burned  at  higher  heats  and  not 
protected  from  the  immediate  contact  of  the  flame,  sur- 
fa*ce  discolorations  are  likely  to  occur  from  the  flying 
ash  of  the  firings.  Where  this  is  a  matter  of  serious 
objection,  it  is  necessary  to  burn  with  fuel  gas  or  crude 
petroleum,  or  at  least  with  a  solid  fuel  containing  less 
iron  than  the  ash  of  our  commoner  coals.  Where  the 
temperature  reached  is  not  so  great  as  to  destroy  the 
sulfates  of  the  clay  and  fix  their  bases  as  undecomposa- 
ble  silicates,  or  where  the  burning  is  conducted  with  fos- 
sil fuels,  containing  sulfur,  with  a  practically  continu- 
ous oxidizing  flame,  so  that  the  clay  takes  up  sulfuric 
anhydrid  from  the  fire  gases,  soluble  salts  are  lodged 
in  the  ware.  These  tend  seriously  to  deface  it  if,  as  in 


FI/)OR-TlIv£   AND   TERRA-COTTA.  155 

the  case  of  bricks  and  terra-cotta,  it  is  exposed  in  outer 
walls  to  alternate  damp  and  drying.  Through  these 
processes  the  salts  are  brought  to  the  surface  as  a  white 
crystalline  efflorescene  in  dry  weather,  which  is  not 
washed  off  but  returned  by  absorption  to  the  ware  with 
the  first  succeeding  rainfall.  The  efflorescences,  con- 
sisting mainly  of  potassium,  calcium,  and  magnesium 
sulfates,  cannot  be  removed  except  by  scraping  the  sur- 
face after  a  period  of  drought  has  brought  them  thor- 
oughly out  of  the  body  of  the  ware.  It  is  then  possible, 
by  careful  application  ot  a  dilute  solution  of  barium 
chlorid,  to  fix  what  still  remains  in  the  body  of  the  wall. 

All  the  efflorescences  of  salts  found  on  walls  are  by  no 
means  derived  from  the  brick  or  terra-cottas  alone. 
Frequently  the  mortar  or  cement  setting  is  responsible  ; 
often  in  stables  or  outhouses,  where  nitrogenous  liquids 
are  absorbed  by  the  brick  or  ammoniacal  fumes  are 
plentiful,  the  ware  may  be  the  seat  of  a  true  nitrif active 
process  and  the  salty  efflorescences  contain  considerable 
nitrates. 

Soluble  lime  salts  may  be  introduced  into  clays  by  the 
materials  used  to  produce  certain  shades  of  color.  Thus 
for  chocolates,  browns,  black,  recovered  manganese, 
and  umber  are  frequently  employed.  The  latter  is  sel- 
dom free  from  gypsum. 

The  physical  properties  of  the  finished  wares  are  re- 
ceiving greater  attention  than  formerly,  though  the  re- 
quirements of  architects  should  be  more  rigorous. 

It  is  very  important  that  brick  and  terra-cottas  be 
burned  to  such  density  that  they  absorb  very  little  water 


156  THE   CHEMISTRY   OF   POTTERY. 

and  suffer  no  decay  through  the  rigor  of  our  northern 
climate. 

Roofing  tile,  which  on  account  of  the  light  roof -con- 
struction long  in  vogue  in  the  United  States,  have  but 
recently  been  introduced  in  larger  amount,  with  sub- 
stantial building,  both  on  account  of  their  technical  ex- 
cellence and  decorative  character,  should  be  particularly 
dense  and  resistant  to  weathering  influences. 

The  same  applies  to  paving  material.  Sidewalk  blocks 
and  paving-brick  are  now  frequently  required  to  show 
less  than  one  per  cent,  absorption  of  water  by  weight. 

Floor-tile  are  not  subjected  to  as  severe  conditions,  but 
if  they  do  take  up  more  than  from  three  to  four  per  cent. 
by  weight  of  water,  they  become  difficult  to  keep  clean 
from  the  grinding  of  dirt  into  their  pores. 

The  following  data,  averaged  from  a  considerable  num- 
ber of  tests  made  on  commercial  wares  of  all  makers  rep- 
resented in  our  market,  will  show  that  while  there  is 
much  that  amply  fills  all  requirements,  the  average  by 
no  means  does. 

WATER  ABSORPTION  OF  FI,OOR-TIIVE. 

Percentages  by  weight. 
Color  of  the  clay.  Extremes.  Average. 

Salmon 1.5  to    9.1  5.8 

Buff i. 9  to    7.2  4.6 

Light  gray 1.9  to    8.5  5.8 

Dark  gray 2.0  to    5.8  4.4 

Chocolate o.o  to    7.4  4.8 

Red J. 5  to    8.4  6.0 

Black 4.4  to  10.3  7.5 

Fawn 8.3 

In  encaustic  tile,  it  is  important  that  the  clay  consti- 
tuting the  "backing  material"  be  fully  as  dense  as  the 


FLOOR-TILE   AND   TERRA-COTTA.  157 

clays  making  the  design  inlaid  in  the  face.  Where  this 
is  not  observed  the  tile  are  in  danger  of  certain  destruc- 
tion wherever  they  are  exposed  to  wet  and  frost,  as  on 
the  floors  of  open  vestibules  and  verandas. 

The  process  of  destruction  is  quite  the  same  as  that 
already  pointed  out  in  the  case  of  porous  glazed  brick, 
subjected  to  similar  exposure.  A  porous  "  backing- 
clay"  takes  up  considerable  water,  which  on  freezing 
does  not  find  room  for  expansion  through  a  nearly  im- 
pervious surface.  In  consequence  these  nearly  vitreous 
surf  ace- clays,  making  the  design  are  very  soon  spalled 
off,  through  ice-pressure.  This  is  particularly  the  case 
in  ware  made  by  the  English  plastic  process.  In  this 
the  surface  bodies,  known  as  "Jaspers"  are  very  dense, 
while  the  backing,  to  make  it  as  little  liable  to  warp  as 
possible,  is  made  very  porous  by  the  addition  of  a  large 
proportion  of  fired  clay  in  coarse  granules  ("grog"). 
Exposed  to  the  conditions  indicated,  the  tile  are,  in  the 
course  of  one  or  two  winters,  ruined  completely. 

Prime  requirements  of  paving-ware  are,  further,  hard- 
ness and  toughness.  It  should  certainly  not  be  possible 
to  scratch  anything  for  such  use  with  even  hardened 
steel.  Grinding  tests  and  tumblings  for  a  certain  length 
of  time  in  a  "rattler,"  as  is  used  in  the  foundries  for 
cleaning  castings,  have  been  made,  with  a  view  to  getting 
factors  representing  both  of  the  necessary  qualities.  The 
results  obtained  are  valuable,  but  mainly  comparative, 
and  no  standards  have,  as  yet,  been  set  up.1 

1  Geological  Survey  of  Ohio,  vol.  vii,  part  i,  p.  192.  Edward  Orton,  Jr.  : 
Clay-working  Industries. 


CHAPTER  XIV. 
REFRACTORY  MATERIALS. 


HK  subject  of  refractory  materials  is 
receiving  much  serious  attention  from 
engineers,  and  the  quality  of  such  prod- 
ucts, furnished  for  the  various  metal- 
lurgical operations,  is  improving  with 
rapid  strides.  Recognition  that  the 
term  refractory  is  altogether  relative  has  gained  ground 
and  leading  to  a  more  exact  formulation  of  demands,  is 
becoming  the  proper  basis  of  improvement. 

Refractory  bodies  being  used  as  apparatus  and  con- 
tainers for  materials  and  products  subjected  to  various 
mechanical  and  chemical  operations  at  high  tempera- 
tures must,  above  all  things,  be  infusible  at  the  temper- 
atures to  which  these  are  to  be  subjected ;  they  must 
not  soften,  swell  up,  or  otherwise  lose  shape  on  fre- 
quent repetition  of  this  condition  of  temperature  or  such 
excess  of  it  as  the  accidents  of  operating  may  make  lia- 
ble. Refractoriness  to  temperature  very  much  beyond 
this  point  is  not  only  useless  but  is  often  bought  at  the 
expense  of  other  qualities,  which  from  this  point  on,  be- 
come quite  as  essential.  Such  qualities  are  the  ability 
to  resist  abrasion,  chemical  action  of  the  material,  or  the 
fuel  and  disintegration  by  repeated  heating  and  cooling. 
It  is  not  possible,  in  the  compass  of  this  work,  to  treat 


REFRACTORY   MATERIALS.  159 

of  the  subject  of  refractory  materials  other  than  as  the 
potter  himself  makes  use  of  them  for  the  building  of 
kilns,  the  making  of  saggars  for  holding  ware  that 
must  be  protected  from  direct  contact  with  the  flame 
and  the  wads  for  luting  the  same  against  the  entrance 
of  fire  gases.  The  potter,  then,  has  nothing  to  do  with 
any  materials,  other  than  clays,  belonging  to  this  cate- 
gory. The  problems  concerning  him,  in  this  connec- 
tion, are  those  relating  to  the  resistance  to  temperature, 
which  is  seldom  as  high  as  in  many  metallurgical  opera- 
tions ;  resistance  to  disintegration  under  the  movements 
of  repeated  heatings  and  coolings,  and  under  the  burden 
of  often  very  great  weights,  but  of  no  abrasive  action, 
as  in  the  blast-furnace  and  lime-kiln  ;  resistance  to  the 
action  of  the  flame  and  the  fluxing  properties  of  the  ash 
of  the  fuel,  but  of  no  strong  chemical  influences  requir- 
ing decidedly  basic  or  entirely  acid  refractory  bodies. 
Nevertheless,  inattention  to  the  demands  of  the  condi- 
tions, although  these  are  not  as  stringent  as  in  most 
chemical  and  metallurgical  operations,  and  are,  perhaps, 
for  this  very  reason  more  often  neglected,  becomes  the 
cause  of  a  serious  drain  on  many  establishments,  in  the 
incessant  repair  of  kilns  and  the  loss  of  saggars. 
Practical  trials,  so-called,  are  usually  quite  insufficient 
to  draw  from  them  conclusions  as  to  the  behavior  of  the 
clay  on  a  working  scale.  Frequently  the  potter  merely 
places  a  lump  of  the  clay,  that  he  intends  to  apply  for  re- 
fractory purposes,  on  the  bag-wall  of  the  kiln  or  in  the 
"  cut  "  of  the  fire,  and  notices  if,  after  its  baking  in  the 
place  of  most  intense  heat,  it  remains  without  trace  of 


160  THE    CHEMISTRY   OF   POTTERY. 

fusion  or  even  sintering.  Often  he  may  even  make  a 
brick  or  saggar  of  the  clay  and  expose  it  similarly  to 
the  most  intense  heat  of  his  kiln,  believing  that  he  is 
thus  making  a  trial  under  severe  working  conditions. 
This  is,  of  course,  far  from  being  the  case.  Only  when 
a  large  number  of  pieces  are  tested  continuously  under 
actual  conditions  of  work,  are  the  accidental  irregulari- 
ties of  the  making  of  single  pieces  lost  in  an  average  ap- 
proximating the  truth  and  only  subjection  to  the  many 
destructive  influences  of  continued  use,  can  answer  the 
question  of  serviceableness.  Such  practical  trials,  if 
properly  carried  out,  are,  contrary  to  the  common  belief 
of  those  unaccustomed  to  systematic  experiment,  expen- 
sive and  long  in  giving  a  conclusive  answer. 

The  question,  whether  the  most  serviceable  refractory 
materials  are  being  employed,  is  an  open  one  in  most 
potteries,  which  seldom  reaches  a  satisfactory  answer  or 
is  dismissed  from  all  consideration,  and  the  losses  en- 
tailed are  accepted  as  inevitable,  without  attaining  the 
assurance  that  they  are  so.  A  knowledge  of  the  chem- 
ical composition  of  the  fire-clays  and  their  pyrometric 
value  by  direct  determination  would  dispel  all  such 
doubts  and  enable  their  selection  on  the  most  rational 
basis  that  the  circumstances  will  allow.  In  addition  to 
an  exact  knowledge  of  the  character  of  the  material,  it 
is  important  to  satisfy  one's  self  that  the  ware  made  of 
it,  the  bricks,  tile,  saggars,  etc.,  have  been  burned  at 
a  temperature  higher  than  that  to  which  they  are  after- 
wards likely  to  be  subjected,  in  order  to  prevent  the  al- 
most certain  danger  of  cracking  and  the  sinking  of 


REFRACTORY   MATERIALS.  l6l 

arches  in  fire-brick  structures,  when  ware  shrinks  on 
being  subjected  in  use  to  temperatures  higher  than  those 
at  which  it  was  originally  burned. 

To  this  important  matter  very  little  attention  has  been 
paid.  Engineers  have  never  as  yet  made  a  specific 
demand  of  the  manufacturers  of  fire-brick  and  tile,  that 
the  ware  for  different  purposes  be  subjected  to  tempera- 
tures which,  according  to  their  use,  must  not  fall  below 
certain  definitely  prescribed  minima. 

Furthermore,  potters  very  seldom  burn  their  saggars 
in  a  special  fire  at  heats  higher  than  they  will  meet  in 
future  use,  but  burn  the  green  stock  continuously  in 
their  regular  ware-kilns,  placing  it  on  top  of  the  bungs, 
where  it  usually  gets  less  heat  than  in  the  places  where 
the  saggers  are  later  likely  to  be  exposed. 

Experience  has  shown  that  in  order  to  resist  repeated 
heatings  and  coolings  most  successfully,  refractory  wares 
must  remain  as  porous  or  even  spongy  in  character  at 
the  temperatures  reached,  as  is  consistent  with  the 
necessary  firmness  of  body  to  carry  the  weights  they 
have  to  bear.  This  is  particularly  the  case  where  from 
position  in  a  furnace  or  elsewhere  the  heating  is  of 
necessity  rapid  and  the  heated  surfaces  are  liable  to  be 
struck  by  cold  air.  In  order  to  attain  this  structure  it 
is  customary  to  mix  with  the  plastic  fire-clay,  which 
should  in  itself  be  so  refractory  as  to  burn  quite  porous 
at  the  heat  given,  at  least  an  equal  weight  of  the  same 
or  a  similarly  refractory  clay  which  has  been  burned 
and  ground  and  so  sifted  that  only  particles  from  the 
size  of  split  peas  to  beans  are  used  ;  the  finer  material 


162  THE   CHEMISTRY   OF   POTTERY. 

and  dust,  which  would  fill  up  the  pores  and  interstices, 
producing  too  compact  a  mass,  are  discarded.  This 
material  is  known  to  potters  as  "grog."  About  pot- 
teries there  are  always  sufficient  broken  fire-brick  and 
saggars  to  supply  the  "grog"  needed.  In  the  fire- 
brick yards  it  is  almost  entirely  substituted  by  coarsely 
ground  unburned  flint-clays.  For  the  best  products  it 
is  sometimes  burned  before  use. 

Clays  markedly  siliceous  remain  porous  even  in  the 
higher  heats  reached  in  pottery  burning  and  can  be 
given  an  open  structure  by  the  addition  of  pure  sili- 
ceous sand,  where  they  do  not  already  contain  it  in 
sufficient  amount  to  bum  of  that  character  in  themselves. 
The  majority  of  refractory  clays  at  command  contain 
more  or  less  finely  divided  free  silica  and  are  not  only  of 
ample  heat-resisting  power,  but  some  of  these  are  of 
notable  quality.  Yet  the  reverse  of  the  popular  belief, 
that  the  refractoriness  is  in  proportion  to  the  free  silica 
present,  is  true :  a  belief  unfortunately  kept  alive  by  the 
statements  in  many  economic  geologies.  The  origin  of 
this  mistaken  belief  is,  that  in  the  comparatively  low 
heats  of  pottery  kilns  the  greater  the  portion  of  uncom- 
bined  silica,  whether  occurring  naturally  in  a  clay  in 
which  the  basic  oxids  are  not  excessive,  or  added  arti- 
ficially, the  more  porous  and  seemingly  more  refractory 
it  is.  Seger,  however,  has  shown1  that  the  syste- 
matic addition  of  silica  to  a  pure  silicate  of  alumina 
Al2O3.2SiO2,  whether  artificial  or  a  natural  kaolinite, 
reduces  the  melting-point  until  the  proportion  A1,O8. 

1  Thoniudustrie-Zeitung,  1893  p.  391. 


REFRACTORY    MATERIALS.  163 

iySiO2  is  reached,  which  melts  with  the  lowest  members 
of  the  scales  which  Seger  and  Bischof  have  severally  set 
up  as  refractory  standards.  Beyond  this  point  the  addi- 
tion of  silica  causes  a  uniform  rise  in  refractoriness  until 
the  proportion  of  alumina  is  so  small  that  it  practically 
disappears  from  the  mixture :  proving  the  high  charac- 
ter of  both  kaolin  and  silica  separately,  as  refractory 
agents  and  the  degree  of  quality  lost  in  their  mixture. 
It  must  not  be  thought  that  because  a  clay  low  in  basic 
oxids  in  proportion  to  the  contained  free  silica,  remains 
porous  in  even  the  hardest  pottery  fires,  that  for  the 
refractory  products  used  by  the  potter,  the  more  silic- 
eous his  fire-clays  are,  the  better ;  particularly  as  the 
more  siliceous  clays  are  much  cheaper  and  as  the 
use  of  a  sandy  clay  or  the  addition  of  quartz  sand  to  his 
fire-clay  will  give  the  necessary  open  structure  as  well 
as  the  more  expensive  "  grog." 

Practical  experience,  where  made  in  conjunction  with 
a  knowledge  of  the  composition  of  the  fire-clays,  has 
proven  that  this  is  not  the  case.  Mechanical  reasons, 
mainly,  unfit  very  siliceous  clays  for  use  as  saggars  ; 
chemical  reasons,  in  many  cases,  make  them  unserviceable 
for  the  fire-brick  of  the  kilns.  It  has  already  been 
pointed  out  that  the  designating  of  a  material  as  refrac- 
tory is  properly  being  done  more  and  more  in  a  relative 
sense,  with  a  statement  as  to  the  conditions  under  which 
it  is  refractory.  No  chemist  or  engineer  would  for  a 
moment  think  of  lining  a  lime-kiln  with  dinas-brick. 

In  nearly  all  treatises  accessible  to  the  manufacturer 
and  the  public  at  large,  however,  refractoriness  is  still 


164  THE   CHEMISTRY   OF   POTTERY. 

treated  as  if  it  were  an  absolute  quality,  inherent  in  the 
material  itself  and  without  discussion  of  the  extraneous 
conditions  that  may  break  it  down.  To  value  a  clay 
directly  and  solely  by  its  "oxygen  ratio"  calculated 
from  the  analysis,  so  commonly  taught  the  public  by 
chemists,  interesting  themselves  in  the  subject  is,  to  say 
the  least,  very  misleading.  All  that  this  ratio  is  sup- 
posed to  teach,  the  relative  fusibilities  of  clays,  is  more 
expeditiously  and  accurately  determined  by  direct 
fusion  in  the  Deville  forge  ;  for  the  structure  of  a  clay 
plays  quite  as  important  a  part  in  its  fusibility  as  its 
chemical  composition. 

The  chemical  analysis  gives  a  general  but  not  a  very 
accurate  clue  to  its  fusibility  ;  but  it  gives  very  positive 
information  of  the  behavior  of  the  clay  toward  out- 
side chemical  agents  to  which  it  may  be  subjected 
under  the  influence  of  heat.  As  far  as  any  danger  from 
fusing  is  concerned,  any  highly  siliceous  clay  reasona- 
bly low  in  basic  oxids,  would  sufficiently  bear  a  pot- 
tery fire  in  itself.  But  the  loss  that  the  potter  fears  in 
his  saggars  is  not  from  fusing  but  from  cracking. 
Their  walls,  compared  with  brick,  are  comparatively 
thin  and  being  filled  with  ware  and  piled  in  "  bungs  " 
to  the  height  of  fifteen  feet  and  more,  often  have  to  bear 
considerable  weight.  While  a  brick,  forming  part  of  a 
solid  wall  of  masonry,  presents  but  one  front  to  the  fire, 
saggars  are  surrounded  with  the  fire-gases  and  upon 
cessation  of  the  fire,  with  the  strong  draught  of  the 
cooling  air.  They  are  therefore  subjected  to  more  or 
less  rapid  changes  of  temperature,  particularly  when 


REFRACTORY   MATERIALS.  165 

• 

placed  opposite  the  kiln-mouths  and  spyholes.  A  sag- 
gar made  of  clay  burning  dense  at  the  heat  of  the  kiln, 
will  not  remain  intact  under  such  conditions  without 
cracking,  and  one  of  a  siliceous  clay,  even  when  left 
quite  porous  in  the  fire,  soon  succumbs  also,  particularly 
if  a  portion  of  the  silica  is  in  the  form  of  grains  of  quartz. 
It  is  probable  that  the  well-known  property  of  the  swell- 
ing of  quartz  in  the  fire  produces  strains,  which  under 
the  severe  conditions  the  ware  cannot  stand. 

A  clay  from  Perry  county,  Ohio,  which  has  found  not 
inconsiderable  use  for  making  saggars,  fire-brick,  wads, 
and  other  refractory  wares  for  pottery  use,  has  the  com- 
position : 

The  portion 

insoluble  in 

The  entire  H3SO4  and 

clay  NaaCO3 

Per  cent.  Per  cent. 

Silica 74.93  5I-52 

Alumina 17.19  0.42 

•   Ferric  oxid 0.79  0.09 

Lime 0.29  0.04 

Magnesia •    0.46  0.02 

Alkalies1 i  .61  0.38 

Combined  water 5.44  o.oo 

100.71  52.47 

RATIONAL  ANALYSIS. 

< 

Per  cent. 

Clay  substance 48.24 

Quartz • 49.72 

Feldspathic  detritus 2.75 

100.71 
1  Combining  weights  respectively  34.9  and  34.5. 


1 66  THE   CHEMISTRY   OF   POTTERY. 

PERCENTAGE  COMPOSITION  OF  THE  CIAY  SUBSTANCE. 

Per  cent. 

Silica 48.53 

Alumina 34-77 

Ferric  oxid 1.45 

Lime 0.51 

Magnesia 0.91 

Alkalies 2.55 

Combined  water 11.28 


100.00 

Thirty  per  cent,  of  the  quartz  would  not  pass  a  sieve 
with  eighty  meshes  to  the  inch.  This  clay,  burned  at 
the  heat  of  melting  orthoclase,  gives  a  yellowish  white 
body,  which  is  very  porous  and  shrinks  but  four  per 
cent.  Saggars  made  of  it,  filled  with  heavy  ware,  and 
set  in  high  bungs  have  an  average  life  of  but  three  and 
one-half  burnings,  at  the  melting  temperature  of  feldspar 
(orthoclase). 

The  same  clay,  made  into  brick,  stood  sufficiently 
well  in  many  instances  as  the  lining  of  pottery  kilns, 
but  whenever  brought  in  contact  with  basic  oxids  the 
destruction  was  rapid  and  complete.  Used  in  the 
mouths,  bottom -flues,  and  arches  of  a  muffle-kiln 
for  slip -glazed  stoneware,  fired  with  coal  yielding 
a  ferruginous  ash,  these  parts  broke  down  in  a  few 
firings.  The  fire-arches  of  two  boilers  built  of  them, 
under  which  the  slack  of  a  similar  coal,  high  in  iron 
pyrites,  was  used,  lasted  but  a  few  weeks.  As  the  lin- 
ing of  a  salt-glaze  kiln  they  melted  away  very  fast. 

The  reason  of  the  clays  succumbing  to  the  action  of 
basic  oxids  is  readily  explained  from  the  composition 
of  stoneware  glazes  themselves. 


REFRACTORY    MATERIALS.  167 

It  was  seen  that  the  most  fusible  silicates  containing 
no  reducible  metals  as  lead  or  zinc,  were  those  approx- 
imating the  formula  iRO.  o.5R2O3.4SiO2  and  that 
more  aluminous  compounds  formed  puckered  viscid 
crusts,  but  not  fusible  glasses.  In  this  clay  the  relation 
of  alumina  to  silica  is  as  i.oto  8.34  and  under  the  action 
of  ferrous  oxid  and  of  soda,  it  would  give  fusible  glasses 
of  about  the  composition  iFeO.  o.5A!2O3.  4.i7SiO2,  and 
iNa2O.  o.5A!2O3.  4.17  SiO2,  which  on  running  off  the 
face  of  the  brick  would  constantly  expose  more  of  it  to 
their  destructive  action.  An  aluminous  clay  would  ab- 
sorb some  of  the  base,  forming  in  time  a  superficial 
vitreous  coat,  but  not  a  fusible  glass  :  it  would  remain 
in  place  as  an  effective  protection  to  the  material  be- 
neath. 

The  wads  and  plugs  used  to  support  the  bungs 
of  salt-glazed  stoneware  and  keep  them  separate  during 
burning,  should  not  be  made  of  siliceous  clays  resem- 
bling the  one  above  discussed,  for  the  reason  that  the 
more  readily  such  a  clay  combines  with  the  alkali 
fumes  the  more  will  it  stick  to  the  ware,  from  the  for- 
mation of  glass  at  the  outer  line  of  contact  with  it. 
These  plugs  cannot,  in  this  case,  be  struck  off  without 
more  or  less  damage  to  the  pieces  to  which  they  were 
attached.  Plugs  of  aluminous  clay  remain  dry  and  give 
no  cause  for  sticking. 

Stoneware  potters  using  clays  that  salt-glaze  readily 
are  often  troubled  with  the  rims  of  the  pieces,  that  are 
set  on  each  other,  adhering,  causing  some  breakage 
wrhen  the  ware  is  taken  apart.  Such  as  have  tried  it, 


l68  THE   CHEMISTRY   OF   POTTERY. 

will  have  found  that  the  washing  of  the  surfaces  of  con- 
tact with  flint,  as  is  done  with  saggars  in  the  white- 
ware  potteries,  may  often  aggravate  the  trouble.  The 
reason  for  this  is  patent  from  the  experiences  and  argu- 
ments given,  as  is  also  the  remedy  sought.  China-clay 
or  a  fire-clay,  as  near  the  composition  of  kaolinite  as 
possible,  would  give  a  proper  wash  for  the  purpose. 
On  the  other  hand,  wads  used  for  luting  the  saggars 
holding  ware  that  is  not  burned  in  the  open  fire,  which 
serve  mainly  as  a  yielding  bed  for  each  saggar  as  it  is 
placed  on  top  of  that  last  set  and  enables  the  kilnman  to 
run  his  "  bungs"  up  straight,  should  be  as  siliceous  as 
possible.  Made  of  such  clays  they  will,  by  their  lower 
shrinkage  and  binding  action,  from  their  remaining 
more  open  in  a  fire  free  from  basic  fumes,  adhere  to  and 
break  the  saggars  less  than  other  clays.  For  such 
purpose  the  clay  cited  proved  excellent. 

The  example  of  this  material  sufficiently  illustrates 
the  value  of  the  chemical  analysis  in  estimating  the 
suitableness  or  unsuitableness  of  a  clay  as  a  refractory 
material  under  different  conditions,  entirely  aside  from 
its  ability  to  bear  mere  temperature  without  fusing. 
Use  of  refractory  materials  is  still  made  with  most  un- 
necessary waste,  both  in  the  employment  of  clays  that 
are  too  good  and  of  those  which  are  altogether  inade- 
quate for  their  application.  The  responsibility  for  this 
must  rest  largely  with  the  very  one-sided  study  that 
chemists  have,  as  a  rule,  made  of  fire-clays,  in  looking 
solely  to  their  relative  fusibilities  and  failure  to  instruct 
the  public  in  the  physical  and  chemical  conditions  that 


REFRACTORY   MATERIALS.  169 

a  refractory  material  has  to  resist  in  different  places  of 
its  application,  which  often  are  of  such  importance  that 
the  relative  fusibilities  are,  beyond  a  certain  point,  of 
very  minor  significance. 

The  best  plastic,  clays  for  saggar-making  have  thus 
far  proven  to  be  the  tertiary  clays  of  New  Jersey , *  par- 
ticularly those  that  with  not  too  high  a  percentage  of 
uncombined  silica  contain  considerable  organic  matter 
that  burns  out  in  the  fire  giving  the  clay  a  very  porous 
structure.  Clays  of  low-binding  power  when  dry,  how- 
ever, as  New  Jersey  clays  are  apt  to  be,  must  be  worked 
very  carefully  that  the  green  ware  does  not  crack  before 
it  comes  into  the  fire. 

The  flint-clays  of  south-eastern  Ohio  form,  when 
burned,  an  excellent  material  as  "grog."  Some  of 
these  are  very  pure,  having  almost  the  composition  of 
kaolinite,  as  the  following  : 

Per  cent. 

Silica 46.54 

Alumina. 38.47 

Ferric  oxid 0.77 

Lime 0.29 

Magnesia 0.23 

Alkalies' i  .38 

.   Combined  water 12.98 

100.66 

The  clay,  in  spite  of  its  composition  was,  when  very 
finely  pulverized,  extremely  difficult  to  decompose  with 

1  For  a  valuable  description  of  New  Jersey  clays  see  "  Report  on  the  clay 
deposits  of  Woodbridge,  South  Amboy,  and  other  places  in   New  Jersey." 
1878,  by  George  H.  Cook,  State  Geologist. 

2  Combining  weight,  40,8. 


170  THE   CHEMISTRY   OF   POTTERY. 

sulfuric  acid.  Even  after  a  treatment  of  fifty  hours  with 
the  acid,  it  left,  after  washing  with  sodium  carbonate,  a 
residue  of  7.54  per  cent. 

From  the  fact  that  in  many  instances  these  flint-clays 
are  more  refractory  than  the  available  plastic  clays  used 
as  the  bonds  in  the  fire-clay  products,  and  that  in  con- 
sequence fire-brick  are  commonly  graded  into  first  and 
second  qualities,  as  they  contain  a  greater  or  less 
amount  of  flint-clay,  the  idea  has  become  rather  com- 
mon that  all  flint-clays  are  highly  refractory.  This  is 
far  from  being  the  case  ;  not  a  few  of  low  quality,  have 
from  this  mistaken  idea,  come  extensively  into  use. 

An  example  of  one  so  used  is  a  flint-clay  of  Beaver 
county,  Pennsylvania,  of  which  the  following  is  the 
analysis : 

The  entire  clay  The  portion  insoluble 

contains  "      in  HaSO4  and  Na2CO3 

Per  cent.  Per  cent. 

Silica 65.85  36.86 

Alumina 22.87  2.19 

Ferric  oxid 1.14  o.  n 

Lime 0.53  0.42 

Magnesia 0.37  0.07 

Alkalies1 2.01  0.47 

Combined  water 6.93  .... 


99.70  40.12 

RATIONAL  ANALYSIS. 

Per  cent. 

Clay  substance 59-5$ 

Quartz 30.42 

Feldspathic  detritus • 9.70 

99.70 


1  Combining  weights  respectively  45.5  and  39.7. 


REFRACTORY    MATERIALS. 
PERCENTAGE  COMPOSITION  OF  THE  CLAY  SUBSTANCE. 

Per  cent. 

Silica 48.66 

Alumina 34-?i 

Ferric  oxid 1.73 

Lime 0.18 

Magnesia o.  50 

Alkalies 2.59 

Combined  water 11 .63 


100.00 

In  a  kiln  burned  to  the  melting  of  feldspar  the  clay 
becomes  very  dense  and  it  melts  in  the  Deville  furnace 
together  with  Seger's  pyrometric  cone  twenty-six,  so 
that  it  would  be  looked  upon  as  a  fire-clay  of  the  lowest 
grade. 

According  to  the  oxygen  ratio  (Feuerfestigkeitsquo- 

O  in  A12O31 

tient)  of  Bischof       ol^So       which  is  1.24  in  this  clay, 

O  in  A12O3 

it  is  not  even  worthy  of  being  designated  a  fire-clay,  as 
the  lowest  member  of  the  scale  he  admits  to  this  clas- 
sification is  the  clay  of  Niederpleis  on  the  Sieg,  with  the 
ratio  1.64. 

In  the  examination  of  a  fire-clay,  first  a  chemical 
analysis  is  required  to  determine  how  it  will  resist 
chemical  and  mechanical  influences  incident  to  its  use. 
From  this  the  oxygen  ratio  (Feuerfestigkeitsquotient) 
may  be  calculated,  according  to  Bischof,2  to  give  a  gen- 

1  Using,  however,  the  old  formulas  and  equivalent  weights  and  calculating 
iron  as  ferrous  oxid  with  the  RO  elements. 

2  C.  Bischof,  Die  feuerfesten  Thone,  Leipzig,  1876 ;    Transactions  of  the 
American  Institute  of  Mining  Engineers,  Virginia  Beach  meeting,  February, 
1894  ;  H.  O.  Hofman  and  C.  D.  Demoud  :  Some  Experiments  for  Determining 
the  Refractoriness  of  Fire-clays. 


172  THE    CHEMISTRY   OF   POTTERY. 

eral  idea  of  its  relative  fusibility.  The  only  value  of 
this  is  to  be  able  to  institute  a  comparison  of  the  clay 
with  others  of  which  the  analyses  are  available,  but  of 
which  no  direct  determinations  of  fusibility  are  recorded. 
Bischof's  quotient  is  preferable  because  it  is  best  known 
and  because  no  other  can,  after  all,  bring  in  the  main 
factor  of  variation,  which  depends  on  the  physical  struc- 
ture of  the  clay. 

Next,  the  direct  determination  of  the  relative  fusi- 
bility of  the  clay  is  made.  It  is  most  satisfactory  if  the 
standing  of  the  clay  is  referred  to  Seger's  pyrometric 
scale.  This  direct  determination  is  made  according'  to 
Seger  and  Cramer1  by  forming  small  tetrahedra  of  the 
clay  to  be  examined,  two  centimeters  high  by  one  centi- 
meter on  the  edges  of  the  triangular  base  and  inclosing 
these,  together  with  similarly  formed  pyrometers  of  the 
higher  members  of  Seger's  pyrometric  scale,  in  a  small 
covered  crucible,  five  centimeters  high  by  four  and  one- 
half  in  diameter,  the  walls  and  cover  being  five  millime- 
ters thick.  The  material  of  which  these  crucibles  are 
made  is  a  mixture  of  equal  parts  of  alumina  and  china- 
clay,  burned  to  a  "grog,"  in  as  high  a  heat  as  is  at 
command  ;  for  forming,  the  material  is  rendered  plastic 
by  the  addition  of  the  necessary  amount  of  unburned 
china-clay.  In  the  bottom  of  the  crucible  a  layer  of  the 
alumina-china-clay  grog,  powdered  fine,  is  packed  and 
the  bases  of  the  trials  pressed  into  it,  to  prevent  their 
being  upset.  The  crucible  with  its  trial  pieces  is  set  in 

1  Thonindustrie-Zeitung,  1893,  p.  1281.  Also,  Transactions  of  the  American 
Institute  of  Mining  Engineers,  Florida  Meeting,  1895.  H.  O.  Hofmau  :  Fur- 
ther Experiments  for  Determining  the  Fusibility  of  Fire-clays. 


REFRACTORY   MATERIALS.  173 

the  Sefstrom  or  Deville  furnace  upon  a  block  of  refrac- 
tory material  of  sufficient  height  to  raise  it  into  the 
focus  of  the  heat  generated  by  gas-retort  graphite  in 
pieces  the  size  of  a  hickory-nut  and  maintained  by 
a  gentle  blast  from  a  rotary  blower. 

The  member  of  the  pyrometric  scale  melting  coin- 
cidently  with  the  clay  being  tested  gives  the  numerical 
index  of  its  refractoriness.  As  the  trial  pieces  are  not 
under  observation  during  the  heating  several  tests  at 
greater  and  less  heats  are  necessary  to  fix  the  right 
degree. 

The  composition  of  these  higher  pyrometers  of  Seger 
is  as  follows  :! 

Cone  number.  Chemical  composition. 


}  "A1A.     44SiO,. 


a 

"     54SiOs. 


'-     66SiO8. 
26'         :  -2^0'.     73810,. 


1  Thonindustrie-Zeitung,  1893,  p.  1252. 


174  THE    CHEMISTRY   OF   POTTERY. 

Cone  number.  Chemical  composition. 

28.  A12O3.  ioSiO2. 

29.  A1203.  8Si02. 

30.  A1203.  6Si02. 

31.  A1203.  5Si02. 

32.  A1208.  4SiOa. 

33-  A12O3.  3SiO2. 

34-  A12O3.  2.5Si02. 

35-  A12O3.  2SiO2.1 
36.  A120S.  2SiO,.2 

To  receive  the  designation  "  fire-clay,"  a  clay  should 
not  melt  more  easily  than  pyrometer  twenty-six. 

High  grade  fire-clays  seem  often  of  low  binding  pow- 
er. While  sufficient  in  this  property,  for  the  making 
of  brick  and  of  the  ordinary  saggars  a  considerable 
amount  of  bonding  clay  is  yearly  imported  from  Germa- 
ny for  the  making  of  glasspots  and  other  large  ware. 
It  would  certainly  seem  that  our  great  wealth  in  clays 
must  include  deposits  having  the  necessary  physical 
qualities  coupled  with  high  refractoriness  and  it  is  im- 
portant that  systematic  examinations  be  made  in  this 
direction  also  the  data  being  furnished  with  the  analy- 
sis and  relative  fusibilities. 

Finally,  in  accordance  with  the  character  of  the  clay 
and  the  use  of  the  products  which  are  to  be  made  of  it, 
the  chemist  should  recommend  the  minimum  tempera- 
ture of  their  burning. 


1  Zettlitz  kaolin,  with  which  also  the  preceding  ones  are  made. 

2  Shale  of  Rakonitz. 


CHAPTER  XV. 
BURNING  THE   WARE. 


HK  important  chemical  operation  to  which 
all  pottery  wares  are  subjected,  is  that  of 
burning  or  firing.  This  operation  is  carried 
on  in  kilns,  that  vary  widely  in  character  and 
construction,  a  variety,  for  which  the  differ- 
ent kinds  of  ware  and  the  questions  of  hand- 
ling and  setting  in  the  kilns,  to  best  advantage,  present 
many  good  reasons.  But  it  cannot  be  denied,  that  in 
many  instances,  the  mechanical  considerations  which 
have  determined  the  style  of  kiln,  have  been  accepted 
without  a  clear  understanding  or  due  appreciation  of  the 
operation  of  burning  in  itself  and  its  effects  on  the  ware. 
These  are  among  the  most  important  of  the  subject. 
The  construction  and  size  of  the  kiln,  the  character  of 
its  fire-places,  and  the  means  of  creating  and  regulating 
the  draught,  must  all  be  such,  that  for  the  purpose  for 
which  the  kiln  is  built,  it  is  possible  : 

1 .  To  obtain  the  necessary  temperatures  required   in 
the  proper  intervals  of  time  and  to  hold  them  at  pleasure, 
as  the  work  may  demand. 

2.  To  get  as  uniform  a  temperature  throughout  the 
kiln  as  possible  by  the  ability  to  direct  of  advance  the 
heat  in  its  different  parts,  as  the  progress  of  the  fire  may 
require. 


176  THE    CHEMISTRY   OF   POTTERY. 

3.  To   be  able  to  get  the  chemical  quality  of  flame 
needed  for  any  operation  at  will. 

4.  To  get  the  maximum  heat  effect  with  a  minimum 
of  fuel. 

From  the  standpoint  of  the  chemist,  who  accepts  the 
methods  and  apparatus  of  empirical  work,  but  must 
modify  or  add  to  them,  so  as  to  eliminate  the  elements 
of  chance,  the  first  consideration  in  firing,  bears  on  the 
question  of  production  and  regulation  of  the  draught. 
As  the  ultimately  high  temperatures  produced  in  all 
pottery  kilns  causes  a  sufficiently  rapid  movement  of 
the  gases  to  bring  an  abundance  of  air  into  the  firings 
for  the  combustion,  many  forms  of  kilns  have  no  regular 
chimney.  The  smoke  and  products  of  combustion 
either  escape  from  the  imperfectly  closed  top,  as  in  the 
old  brick-clamp  or  from  openings  in  the  arched  crown, 
as  in  the  English  pottery  kiln.  The  latter,  it  is  true,  is 
as  a  rule,  topped  with  a  cone,  intended  to  collect  the 
gases  from  the  various  openings  and  convey  them  above 
the  roofs  of  the  surrounding  buildings.  It  acts,  in  a 
measure  as  a  chimney  but  having  a  large  radiating 
surface  it  acts  very  poorly,  when  a  positive  draught  in 
the  kiln  is  most  needed,  namely,  when  the  temperature 
in  the  kiln  is  still  low.  Besides,  its  form  precludes  the 
possibility  of  regulation  with  it  except  to  a  very  limited 
extent.  As  the  admission  of  air  into  the  kiln  is  deter- 
mined by  the  speed  with  which  the  products  of  combus- 
tion are  withdrawn  and  by  the  greater  or  less  obstruction 
presented  to  the  air  at  its  entrance;  in  kilns  of  the  above 
type  this  regulation  is  mainly  effected  in  the  latter  way. 


BURNING   THE    WARE.  177 

Such  a  method  of  regulating  the  volume  of  air  needed, 
by  more  or  less  obstruction  at  its  entrance,  is  very 
troublesome  and  uncertain.  There  must  of  necessity  be 
many  places  of  entrance  to  attend  to  affecting  the  draught 
more  or  less  by  their  size,  shape,  and  the  position  of  the 
fuel  in  the  firings.  The  regulation  of  these,  individu- 
ally, must  at  most  be  the  veriest  guesswork.  By  the 
withdrawal  of  all  the  products  of  combustion  through  a 
single  stack,  of  such  dimensions  to  produce  at  all  times 
a  draught  in  excess  of  the  needs,  and  controlling  this 
with  a  single  damper,  positive  in  its  action,  the  pressure 
in  the  kiln  can  at  will  be  brought  to  any  definite  degree 
below  that  of  the  atmosphere  and  produce  a  proportion- 
ate influx  of  air  through  all  openings.  The  individual 
air  inlets  can  then  be  adjusted  separately,  if  need  be,  to 
the  requirements  of  each  fire-place,  or  manipulated  to 
throw  the  flame  to  the  top  or  crowd  it  to  the  center  and 
bottom  of  the  kiln,  as  the  progress  of  the  heat  in  differ- 
ent parts  of  the  kiln  may  require. 

The  draught  in  the  chimney  determines  the  propor- 
tion of  air  entering  the  kiln,  and  therefore  the  character 
of  the  combustion  and  the  quality  of  the  flame. 

Hence,  it  is  necessary  to  determine,  by  analysis,  the 
composition  of  the  gases  leaving  the  kiln  through  the 
stack  and  at  the  same  time  to  note,  by  a  continuously 
acting  kiln-barometer,  the  pressure  in  the  kiln  below 
that  of  the  air,  at  which  the  gases  are  of  the  composition 
found.  A  few  of  such  analyses,  during  the  progress  of 
a  firing,  at  times  when  the  kiln- mouths  have  just  been 
charged  with  fresh  fuel  and  when  the  same  has  well 


1 78  THE   CHEMISTRY   OF   POTTERY. 

burned  down,  will  establish  the  proper  kiln-pressures 
for  admitting  the  needed  amount  of  air  at  the  different 
stages  of  fire,  so  that  the  damper  can  be  handled  alto- 
gether by  the  reading  of  the  kiln-barometer  and  gas- 
analyses  will  only  be  needed  as  occasional  checks. 

For  the  gas  analyses,  the 
Orsat  apparatus  is  conven- 
ient, though  even  a  simple 
Bunte  or  a  Cramer  carbon- 
ic-acid pipette  is  sufficient 
to  gain  the  necessary  knowl- 
edge of  the  quality  of  the 
gases  leaving  the  kiln. 
The  most  convenient  kiln- 
barometer  or  draught  me- 
ter is  a  modfied  form  of 
that  of  Scheurer-Kestner.1 
It  consists  of  a  tin  box 
having  a  glass  guage  tube 
at  the  side,  opening  into 
the  box  at  its  lower  end 
and  into  the  air  at  the 
upper,  inclined  at  an  angle 
of  nine  degrees,  through 

KILN-BAROMETER  OR  DRAUGHT-METER,  which   the   rise  and   fall   of 

the  liquid  in  the  box  is  amplified  ten-fold,  making  the 
readings  of  the  slight  changes  of  level  possible  with 
the  unaided  eye.  The  guage  is  supplied  with  a 


1  Thoniudustrie-Zeitung,  1891,  p.  696 ;    Dr.  Julius  Post:    Chemisch-Tech- 
niche  analyse,  vol.  II,  p.  73  ;  Braunschweig,  1890-1. 


BURNING   THK   WARE.  179 

scale  and  suitable  metallic  fittings  attaching  it  to  the 
box  and  protecting  it  against  breakage.  The  box  is 
filled  with  carbon  oil  to  the  zero  mark  on  the  guage- 
glass,  when  it  hangs  level.  The  opening  through 
which  the  box  is  filled  is  closed  with  a  thumb- 
screw pierced  with  a  piece  of  brass  tubing.  By  means 
of  a  piece  of  rubber  hose  attached  to  this  and  a  porcelain 
tube  walled  air-tight  into  the  kiln  or  chimney,  the  at- 
mosphere of  the  latter  is  connected  with  that  above  the 
oil,  communicating  the  kiln-pressure  to  it.  Instead  of  a 
porcelain  tube  a  duct  may  be  walled  from  the  kiln  to  the 
top  of  the  ' '  hob  ' '  surrounding  the  same  and  the  bell- 
shaped  iron  foot  of  the  stand  of  the  instrument  placed 
over  it.  The  rubber  hose  connection  of  the  kiln-barom- 
eter is  then  attached  to  the  gas-pipe  upright,  as  shown 
in  the  cut,  and  the  communication  with  the  kiln  estab- 
lished through  the  duct,  bell,  upright  pipe,  and  hose  to 
to  the  box.  Through  the  porcelain  tube  used  to  com- 
municate the  kiln-pressure,  where  this  is  used,  the  sam- 
ples of  gas  for  analyses  may  be  taken,  though  it  is  also 
well  to  take  such  samples  from  the  top  and  bottom  of 
the  kiln,  by  other  porcelain  tubes,  properly  let  into  the 
walls  of  the  same  at  the  respective  places. 

Unfortunately,  these  simple  devices  for  attaining  a  posi- 
tive knowledge  of  the  character  of  the  fire  and  helping  it 
in  the  condition  desired  by  regulating  the  damper  with 
reference  to  distinct  stack-pressures,  have  not,  as  yet, 
found  entrance  into  our  potteries.  Firing  is  looked 
upon  as  a  mysterious  operation  and  even  experienced 
burners  do  not  attack  it,  in  new  kilns,  or  with  new 


l8o  THE   CHEMISTRY   OF    POTTERY. 

wares,  with  the  confidence  that  the  possession  of  positive 
data  and  a  knowledge  of  their  meaning  would  give  them. 

The  majority  of  even  the  best  kilnmen  waste  con- 
siderable fuel  by  the  admission  of  excessive  amounts 
of  air  to  the  kiln.  How  great  this  waste  is,  few  potters 
perhaps  realize.  A  glost  kiln  fired  with  crude  petro- 
leum sprayed  into  the  kiln-mouths  with  compressed  air 
from  atomizing  burners  required  1,050  gallons  or  3,135 
kilograms  of  the  fuel.  The  kilo  of  oil  required  about 
14.35  kilos  of  air  for  its  combustion  and  probably  had  a 
calorific  value  of  10,000  heat  units. 

Analyses  of  the  flue  gases  proved  that  throughout  the 
burning  about  175  per  cent,  of  air  was  used,  as  the  kiln 
was  ordinarily  fired.  Subsequent  firings  showed  that 
although  lead  glazes  were  being  burned  an  average 
consumption  of  no  per  cent,  of  air  was  not  only  ample 
for  smokeless  combustion,  but  did  not  endanger  a  loss  of 
ware  by  reduction  of  the  lead  of  the  glaze.  Hence, 
sixty-five  per  cent,  of  the  theoretically  necessary  amount 
of  air  was  admitted  to  the  kiln,  above  its  practical  needs, 
amounting  to  29,239  kilograms.  At  the  finish  of  the 
burning  the  gases  left  the  kiln  with  a  temperature  suffi- 
cient to  melt  an  alloy  of  twenty  per  cent,  silver,  eighty 
per  cent,  gold,  or  about  at  the  temperature  1,045°  C. 
The  average  heat  of  the  gases  leaving  the  kiln  from  be- 
ginning to  end  of  the  fire  was  therefore  523°  C.  Tak- 
ing 0.2377  as  the  specific  heat  of  air,  each  kilogram  of 
the  excess  of  the  same  took  away  needlessly  124.32  heat 
units  or  3,634,660  in  all:  a  quantity  which  required 
the  combustion  of  122  gallons  of  the  petroleum  to  sup- 


BURNING   THK   WARE.  l8l 

ply.  At  subsequent  firings,  a  careful  regulation  of  the 
draught,  according  to  the  practical  needs,  effected  a 
saving  of  about  three  barrels  of  oil  to  each  burning. 

The  above  example  is  by  no  means  an  extreme  case  ; 
on  the  contrary  it  is  probable  that  in  coal-firing  an  ex- 
cess of  upwards  of  100  per  cent,  of  air  is  the  practice. 
In  an  ordinary  boiler-firing  it  is  customary  to  allow  an 
excess  of  thirty  per  cent,  of  air,  in  order  to  insure  am- 
ple contact  of  the  solid  fuel  with  the  same,  on  the  grate- 
bars.  In  firing  a  pottery  kiln,  the  excess  of  air  may  be 
less,  because  of  the  better  opportunity  for  combination 
in  the  greater  distance  of  travel  under  far  higher  tem- 
peratures than  in  the  boiler,  though  how  much  less,  is  a 
matter  of  experience,  depending  upon  the  fuel,  the  kiln 
temperature,  the  character  of  the  firings,  and  the  dis- 
tance the  gases  have  to  travel  in  the  kiln,  before  reach- 
ing the  exit. 

In  order  to  distribute  the  heat  as  uniformly  as  pos- 
sible throughout  the  kiln,  it  is  important  that  the  com- 
bustion take  place  mainly  beyond  the  fire-places  and 
that  these  act  rather  as  generators.  Those  parts  of 
the  kiln  exposed  to  radiation  from  the  fire-places, 
known  as  the  "cuts,"  are  invariably  hotter  than  the 
others  and  this  condition  is  greatly  aggravated,  when 
the  fuel  lies  shallow  and  is  mainly  consumed  on 
the  grate-bars.  The  heating  of  the  kiln  does  not,  in 
this  case,  it  is  true,  take  place  entirely  by  radiation  and 
conduction,  for  through  the  dissociation  of  the  products 
of  combustion  in  passing  this  heated  zone,  combustible 
gases  are  formed  which  recombine  at  the  lower  tempera- 


1 82  THE    CHEMISTRY   OF   POTTKRY. 

tures  in  the  interior  of  the  kiln.  But  the  absorption  of 
heat  in  the  ' '  cuts ' '  by  the  dissociating  gases  is  never 
sufficient  to  reduce  the  heat  to  a  moderate  difference 
from  that  in  the  rest  of  the  kiln.  For  this  reason,  the 
fire-places,  instead  of  being  broad  and  shallow,  should 
be  narrow  and  deep,  approaching  the  character  of  fuel- 
gas  generators.  The  gases  leaving  them  being  mainly 
reducing  in  character,  burn  in  their  progress  through 
the  kiln,  as  by  diffusion  or  by  striking  solid  objects  ob- 
structing their  way,  they  become  thoroughly  mixed 
with  the  air  entering  through  doors,  air-openings,  and 
spy-holes.  It  is  only  where  the  firings  are  of  this  char- 
acter that  it  becomes  possible  to  burn  with  a  relatively 
small  excess  of  air  and  effect  the  economies  incident 
to  so  doing.  For  where  the  fuel  lies  shallow  on  the 
grate-bars  air  channels  soon  work  through  it,  admitting 
excessive  amounts  of  air  ascending  in  compact  columns, 
which  afterwards  comes  but  little  into  play  in  com- 
bustion, where  the  firings  send  the  complete  products 
of  combustion  and  not  combustible  gases  along 
with  this  air  into  the  kiln.  As  there  is  in  this  case  ab- 
solutely no  way  of  knowing  the  proportion  of  grate  sur- 
face sending  nearly  pure  air  into  the  kiln,  and  as  there 
is  no  way  of  regulating  it,  a  reduction  of  the  general 
draught  of  the  kiln  until  the  flue-gases  have  the  proper 
composition,  would  often  be  attended  by  a  cutting  down 
of  the  combustion  until  insufficient  for  producing  the 
necessary  rise  in  temperature.  Only  where  the  kiln- 
mouths  send  reducing  gases  into  the  kiln  and  the  needed 
excess  of  air  enters  through  openings  that  can  be 


BURNING   THE   WARK.  183 

manipulated  and  controlled  is  the  fire  absolutely  in 
hand  as  then,  by  closing  the  air  inlets  more  or  less,  the 
draught  of  the  stack  can  be  made  to  act  more  or  less  on 
the  fire,  sucking  the  fuel-gases  in  the  proportion  wanted. 

It  is  a  familiar  fact  to  all  who  have  had  to  make 
analyses  of  furnace-gases,  that  columns  of  gases,  even 
where  great  chemical  affinity  exists  between  them  and 
the  temperature  is  favorable  to  combination,  may  travel 
side  by  side  with  little  diffusion  and  combination.  It  is 
necessary,  therefore,  to  make  the  distance  of  travel  in 
the  furnace  of  sufficient  length  and  provide  that  the  gases 
strike  solid  objects  and  are  compelled  to  turn  on  them- 
selves to  effect  mechanically  as  thorough  a  mixture  as 
possible.  The  most  rational  kilns,  then,  are  those 
working  on  what  is  known  as  the  "  down  draught"  prin- 
ciple. In  these  the  gases  rise  from  the  fire-places  to  the 
crown  against  which  they  strike  and  are  compelled  to 
descend  between  the  bungs  of  saggars  or  of  ware  to  the 
flues  under  the  floor  which  lead  to  a  center  tunnel  con- 
nected with  the  stack.  The  striking  against  the  crown 
of  the  kiln,  the  horizontal  movement  under  the  same, 
with  the  mixture  effected  by  the  impeding  tops  of  the 
bungs  of  ware  and  the  down  ward  movement,  most  effect- 
ually breaks  up  any  tendency  of  the  gases  to  move  in 
separate  channels. 

The  building  up  of  the  ware  in  the  kilns  and  the  ar- 
rangement of  the  flues  and  the  tunnels  to  the  stack  must 
be  made  with  careful  consideration  to  the  end  that  the 
gases  from  the  different  fire-places  meet  similar  fric- 
tional  resistances  throughout  their  long  distance  of  travel ; 


1 84  THE   CHEMISTRY   OF   POTTERY. 

otherwise  the  combustion  in  different  parts  of  the  kiln 
will  vary  so  as  to  make  it  impossible  to  burn  it  with  any 
approximation  to  uniformity.  And,  furthermore,  it 
must  not  be  forgotten,  that  as  the  gases  have  to  travel 
at  least  three  times  the  distance  they  do  in  an  up-draught 
kiln,  and  where  the  stack  is  outside  of  the  kiln,  at  least 
four  times  this  distance,  the  frictional  resistance  they 
encounter  is  proportionately  as  much  greater  and  must 
be  overcome  by  a  proportionately  greater  draught  or  a 
corresponding  reduction  in  the  speed  of  combustion  and 
cooling  will  have  to  be  expected.  This  fact  has  often 
been  overlooked  where  an  up-draught  kiln  has  been 
changed  into  a  down-draught,  by  closing  the  crown 
holes  and  carrying  a  center  stack  in  the  kiln  to  just 
above  the  crown,  the  cone  carrying  off  the  smoke  and 
creating  the  draught  as  before,  expecting  an  increase 
of  this  draught  without  any  alteration.  Where  in 
such  a  case  it  was  attempted  to  burn  glazes  high  in 
basic  oxids,  particularly  sensitive  ones  and  such 
of  a  composition  causing  devitrification  if  they  are  not 
rapidly  cooled,  disappointment  at  the  down-draught 
principle  has  unjustly  ensued.  With  a  suitable  provi- 
sion for  the  necessarily  greater  draught  required  to 
draw  the  fire-gases  through  their  longer  and  more  tor- 
tuous course,  all  the  seeming  advantages  of  the  up- 
draught  kiln  can  be  obtained,  without  sacrifice  of  the 
more  thorough  combustion  aimed  at  in  the  other  con- 
struction. 

The  dissociation  of  the  products  of  combustion  in  the 
"  cuts"  of  the  kiln  as  well  as  the  production  of  combus- 


BURNING   THK   WARE.  185 

tible  gases  in  the  producer-like  fire-places  yields  sufficient 
length  of  flame  for  the  longer  distance  of  travel,  in  the 
down-draught  kiln,  even  with  short-flamed  fuel.  This 
cannot,  however,  be  counted  on  to  extend  the  effect  of 
the  fire  with  practical  uniformity  of  temperature  to  an 
indefinite  distance  in  the  kiln.  As  the  flame,  after  some 
distance  of  travel  toward  the  outlet,  becomes  freighted 
with  the  products  of  combustion,  these,  together  with 
the  inert  gases  that  it  necessarily  carries  with  it,  nitro- 
gen and  the  water  vapor  derived  from  the  moisture  in 
the  fuel  and  the  oxygen  which  it  contained,  may  grow 
to  such  proportions  as  to  extinguish  it,  even  though  it 
still  contains  combustible  products  and  an  excess  of 
oxygen.  There  is  then  a  practical  limitation  to  the  dis- 
tance of  travel  of  the  fire-gases,  consistent  with  their 
being  still  effective,  which  fixes  the  dimensions  of  kilns. 
When  these  exceed  a  capacity  of  3,500  cubic  feet  it  is 
difficult  to  regulate  them  so  as  to  burn  sensitive  products 
uniformly. 

The  regulation  of  the  English  pottery  kilns,  which  are 
the  common  type  in  use  in  most  of  our  ceramic  indus- 
tries, must  be  effected  altogether  by  the  manipulation  of 
the  air  inlets.  In  up-draught  kilns,  the  draught  being 
as  a  rule  much  too  strong,  the  bottoms  are  most  liable  to 
be  hard,  while  in  down-draught  kilns  the  reverse  is  the 
case  and  it  is  often  difficult  to  bring  the  lower  ware  to 
the  required  temperature.  In  the  latter  case  the  flame 
must  be  crowded  to  the  center  and  bottom  of  the  kiln, 
which  is  done  in  admitting  air  over  the  kiln-mouths  by 
withdrawal  of  the  regulating  brick.  The  air  sucked  in 


1 86  THE   CHEMISTRY   OF   POTTERY. 

through  the  exposed  opening  rises  in  a  column  next  to 
the  kiln-wall  to  the  crown  and  forces  the  fire-gases  in- 
ward. Where  this  source  of  air  is  insufficient  the  fire- 
doors  will  often  have  to  be  given  more  or  less  opening. 
Amounts  of  air,  largely  in  excess  of  the  needs  of  com- 
bustion, are  in  this  manner  often  necessarily  introduced 
into  the  kiln  for  regulation,  and  as  previously  pointed 
out,  involve  a  correspondingly  greater  consumption  of 
fuel.  Where  the  bottoms  of  the  kiln  get  too  hard  it  is, 
of  course,  necessary  to  cut  off  as  near  as  possible  all  air 
supplies  over  the  fire,  which  tend  to  crowd  it  down. 
Serious  defects  interfering  with  the  regulation  must  be 
met  by  permanent  alterations  in  the  kiln,  as  increasing 
or  reducing  the  size  of  the  crown  holes,  the  flues,  and  of 
the  cone,  or  of  admitting  air  into  the  latter  over  the 
crown.  But  the  variations  in  draught  due  to  the  wind, 
and  the  changing  pressure  of  the  atmosphere,  and  the 
rise  in  temperature  of  the  kiln,  which  in  a  kiln  with  a 
stack  and  damper  can  be  met  by  the  setting  of  the  lat- 
ter, can  in  these  be  counteracted  by  the  manipulation 
of  the  air  inlets  alone. 

The  rapidity  of  firing  a  kiln  is  determined  by  the 
draught,  the  size  and  number  of  the  fire-places,  the  in- 
tervals of  time  in  which  fuel  is  supplied  to  the  fires  and 
the  weight  and  character  of  the  ware  contained  in  the  kiln. 

Thin-walled  ware,  that  is  reasonably  uniform  in  sec- 
tion, as  most  dish  ware,  can  be  burned  and  cooled  quite 
rapidly  without  danger  of  loss  in  the  kiln  or  creation  of 
a  brittle  product  as  one  with  internal  strains  tending  to 
shatter  it.  It  is  well  to  have  kilns  for  the  manufacture  of 


BURNING  THE  WARK.  187 

this,  with  a  very  strong  draught,  providing  the  kiln- 
mouths  be  correspondingly  large  or  numerous  to  take 
the  fuel  for  a  plentiful  combustion.  It  is  then  possible 
to  push  the  productivity  of  a  kiln,  if  the  manufacture 
demands  it. 

Glazes  having  a  tendency  to  cloud  or  devitrify  must 
be  fired  and  especially  cooled  rapidly.  With  these  it  is 
not  infrequently  the  custom  to  "  draw  the  fires,"  as  it  is 
called.  As  soon  as  the  required  temperature  has  been 
reached  the  grate-bars  are  let  down,  the  fire  raked  out 
of  the  mouths  and  extinguished. 

Sanitary  ware,  terra-cottas,  tile,  and  all  varieties  of 
brick  should  be  fired  and  cooled  slowly.  In  all  of  these 
the  bodies  are  thick  and  opportunity  must  be  given  the 
hygroscopic  and  especially  the  combined  water  of  the 
clay  to  leave  it  slowly  so  as  not  to  incur  the  danger  of 
warping,  shattering,  or  cracking  the  pieces.  It  is  like- 
wise important  that  such  products  be  cooled  slowly, 
whereby  they  undergo  a  sort  of  annealing  process  that 
is  very  essential  to  their  life.  Building  terra-cottas  and 
the  vessels  used  in  sanitary  plumbing  often  have  thin 
walls  attached  to  thick  ones ;  the  unequal  cooling  of 
which  is  unavoidably  associated  with  the  rapid  cooling 
of  non-conducting  products,  brings  about  unequal  and 
severe  strains,  making  these  pieces  sensitive  to  sudden 
changes  of  temperature  and  light  blows  or  jars.  All 
paving  material  is  sensibly  toughened  by  slow  cooling. 
The  kilns  for  these  products  need  not,  therefore,  have 
so  great  a  proportionate  fire-place  volume  to  the  volume 
of  kiln  space  and  the  draught  may  be  correspondingly 


1 88  THK   CHEMISTRY   OF   POTTKRY. 

less.  The  slowing  of  the  fire  by  charging  the  furnaces 
with  baitings  of  fuel  at  greater  intervals  is  only  possible 
with  a  corresponding  reduction  of  the  draught.  Where 
the  fire  is  maintained  with  solid  fuel  the  temperature 
should  rise  in  regular  throbs  or  pulsations  correspond- 
ing with  the  baitings.  These  should  burn  down  to  clear 
glowing  coals,  before  the  mouths  are  again  charged,  but 
not  so  far  that  the  temperature  begins  to  recede  from 
the  point  reached.  The  progress  of  the  heat  must  be 
continuously  upward  and  as  uniform  in  the  periods  of 
its  rising  as  possible.  Where  then  this  progress  is  too 
great  the  fuel  must  be  made  to  burn  more  slowly  by  cut- 
ting down  its  draught. 

The  stormy  evolution  of  gaseous  products,  when  fresh 
baitings  of  fuel  are  put  on  the  fires,  produces  periodic- 
ally a  reducing  atmosphere  of  greater  or  less  duration 
in  the  kiln ;  if  this  is  succeeded  each  time  by  a  clear 
oxidizing  atmosphere,  no  danger  of  the  reduction  of 
even  lead  glazes  need  be  feared.  In  fact,  the  alterna- 
ting of  a  short  period  of  reducing  influences  followed  by 
a  longer  one  of  oxidation  is  in  many  cases  preferable  to 
a  continuously  oxidizing  fire  throughout  the  burning. 
All  fossil  fuels,  oil,  coal,  and  the  fuel-gas  made  from  it, 
as  well  as  natural  gas,  contain  considerable  sulfur, 
which,  on  burning,  is  absorbed  in  an  oxidizing  atmos- 
phere as  sulfuric  anhydride  by  glazed  surfaces,  causing 
unsightly  separations  of  crystalline  sulfates  in  the  melted 
glass. 

Through  the  action  of  reducing  gases,  these  sulfates 
formed  on  the  surfaces  of  the  glaze  are  decomposed,  the 


BURNING   THE   WARE.  189 

sulfur  being  carried  away  with  the  other  gases  of  com- 
bustion as  dioxid. 

Seger1  found  that  the  clay  of  Birkenwerder,  rich  in 
lime  and  iron,  glowed  for  two  hours  in  an  atmosphere  of 
sulfur  dioxid,  and  air  took  up  13.6  per  cent,  of  sulfuric 
anhydrid.  By  heating  it  again  in  a  reducing  atmos- 
phere this  could  be  entirely  expelled.  Through  this 
discovery  he  found  an  explanation  of  the  phenomenon 
that  clays  of  this  type,  which  in  the  ordinary  intermit- 
tent kilns  fired  with  coal  produce  a  yellow  or  cream- 
colored  body  from  the  formation  of  a  lime-iron  silicate, 
in  continuous  or  gas-fired  kilns,  generally  burned  red  or 
were  flashed  with  red.  In  the  latter  the  kiln-atmos- 
phere is  generally  continuously  oxidizing.  Under  this 
condition  the  lime  in  the  clay  absorbing  sulfuric  acid 
from  the  fire-gases  does  not  combine  with  the  iron  and 
leaves  the  latter  to  impart  its  red  color  to  the  product. 
It  has  been  found  that  in  following  Seger 's  suggestion 
of  manipulating  the  draught  in  these  kilns  so  that  at 
regular  intervals  the  kiln-atmosphere  is  strongly  reduc- 
ing, these  clays  can  be  burned  uniformly  to  a  cream  col- 
or as  well  as  in  the  intermittent  kilns  fired  with  solid  fuel. 

In  the  case  of  tin-enamels,  the  writer  has  observed 
that  when  burning  in  kilns  fired  with  crude  petroleum 
sprayed  into  the  mouths  with  compressed  air,  so  that 
the  fire  is  continuously  oxidizing,  they  are  very  liable  to 
have  a  pink  cast,  while  burned  with  an  alternately  re- 
ducing and  oxidizing  flame  they  become  snowy  white. 
These  instances  will  suffice  to  show  that  the  smoke 


1  Thonindustrie-Zeitung,  1877,  p.  22. 


IQO  THE    CHEMISTRY   OF   POTTERY. 

issuing  at  regular  intervals  from  the  ordinary  kiln  is  not 
only  harmless,  but  may  play  a  very  useful  roll,  provided 
that  the  burning  be  so  conducted  that  too  much  fuel  is 
not  wasted  and  the  kiln-temperature  is  not  depressed  in 
throwing  too  much  coal  on  the  fires  at  each  baiting. 

The  useful  results  of  temporary  and  recurring  reduc- 
ing periods  in  the  burning,  unwittingly  attained  in  fir- 
ing with  solid  fuels  can,  of  course,  be  just  as  easily  ob- 
tained with  oil  and  fuel-gas.  But  as  these  are  often 
used,  mainly,  from  a  supposed  advantage  of  the  ease 
with  which  they  are  handled  to  give  a  smokeless  com- 
bustion, and  as  to  the  average  mind  this  is  the  perfec- 
tion of  firing  regardless  of  the  results  sought,  it  is  dim- 
cult  to  make  the  potter  understand  that  this  may  be  as- 
sociated with  practical  objections. 

The  phenomenon  that  potters  commonly  ascribe  to  the 
effect  of  sulfur  in  the  coal  is  due  to  excessive  reduction 
in  glost-kilns,  through  careless  firing.  It  is  a  reduction  of 
the  lead  of  the  glaze  so  that  the  latter  turns  black  and  it 
comes  when  the  fireman  has  been  careless  about  clinkering 
his  fires  and  has  not  kept  the  grate-bars  clean.  The  phe- 
nomenon may  in  so  far  hang  together  with  sulfur  in  the 
coal,  that  inasmuch  as  the  sulfur  is  mainly  present  as 
iron  pyrites,  when  this  is  high,  the  large  amount  of  iron 
slag  or  clinker  that  is  formed  requires  especial  watchful- 
ness, to  prevent  its  shutting  off  the  air  supply  coming 
through  the  live  coals. 

Our  pottery  kilns,  like  all  the  older  apparatus  using 
high  temperatures  that  discharge  their  products  of  com- 
bustion directly  into  the  air,  without  being  compelled 


BURNING   THE   WARE.  IQI 

to  first  give  off  their  surplus  heat,  where  it  can  again  be 
utilized  for  the  burning,  are  under  the  most  favorable 
conditions  of  operation  very  wasteful  of  fuel. 

The  most  successful  application  of  the  regenerative 
principle,  in  which  a  greater  effect  is  obtained  from  fuel, 
than  perhaps,  in  any  other  apparatus,  is  the  ring-brick 
kiln  of  Hoffmann  and  Licht.  Few  of  these  have  as  yet 
been  built  in  the  United  States  and  few  of  the  regenera- 
tive gas-kilns  of  Mendheim  and  of  Dunnachie.  Ignor- 
ance of  the  progress  made  in  ceramic  industries  else- 
where, are  in  part  responsible  for  this,  though  valid 
commercial  reasons  connected  with  the  cheapness  of  our 
fuel  and  the  generally  higher  price  of  labor,  have  made 
people  slow  to  lock  up  the  greater  amounts  of  capital 
involved  in  these  modern  kilns.  There  is  no  reason, 
however,  since  the  progress  made  in  this  direction  by 
the  iron  and  glass  industries,  why  the  pottery  industries 
should  not  follow ;  especially  as  in  these  the  application 
is  much  more  simple. 

Special  ' '  stoves ' '  and  ' '  regenerators  ' '  for  taking  up 
the  waste  heat  and  imparting  it  to  the  air  for  the  com- 
bustion being  unnecessary,  as  a  battery  of  kilns  suita- 
bly connected,  with  their  contained  ware,  act  in  turn  in 
this  capacity ;  the  kilns  in  advance  of  the  one  in  fire 
taking  up  the  waste  heat  from  the  products  of  combus- 
tion before  these  are  turned  into  the  stack,  while  the  air 
for  the  combustion  serves  to  cool  those  which  have  been 
fired  and  comes  hot  to  the  one  burning.1 

1  Groves  and  Thorpe  :  Chemical  Technology,  Vol.  i.,  Fuel  and  its  Appli- 
cations ;  Thonindustrie-Zeitung,  1877-1895. 


INDEX. 


ACETATES  in  white  lead  deleterious  for  preparation  of 

glazes  therefrom 125 

Alloys  of  Prinsep 30 

Analysis  of  clay.  Necessity  for  accuracy  in,  3  ;  separation  of 
silica  and  alumina,  4  ;  alumina  and  ferric 
determination,  5  ;  impurities  in  reagents, 
5  ;  impurities  in  distilled  water,  6  ;  de- 
termination of  potash  and  soda,  6  ;  prox- 
imate analysis,  6  ;  "  clay  substance,"  8  ; 
"  rational  analysis,"  8  ;  calculation  of.  to 
dried  substance,  13  ;  for  white  enameled 
brick,  140,  144 ;  for  saggars,  fire  brick, 

*  etc 165,170 

BANKO  ware 78 

"  Barbotine"  ware 73 

Bischof's  opinion  of  rational  analysis 9 

Biscuit,  analysis  of,  13  ;  calculation  of  mass  for 13 

"  Blue  calx,"  composition  of 120 

Boron  in  glazes,   14  ;  action  of  boron  in  glazes,  50  ;  volatili- 
zation of 125 

Brick,  white  enameled 139 

Broginarts'  classification  of  ceramics 41 

Burning,  175  ;  draught  in  the  kiln,  176  ;  analysis  of  kiln 
gases.  177;  draught  meter,  178;  firing,  179;  uni- 
form distribution  of  heat,  placing  of  the  ware  in 
kilns,  down  and  up  draught  kilns,  183  ;  rapidity  of 
firing,  186  ;  'reducing  action  of  fresh  fuel,  188  ;  re- 
generative kilns 191 

"  C.  C."  ware 43,  117 

Ceramics,  classification  of 41 

"  Checking" 21 

Chemical  ware 78 

China-clay 96 


194  INDEX. 

Chromic  oxid  use  as  coloring  material 133 

Clay  analyses.  Kaolin  from  Nelson  Co.,  Va.,  10 ;  kaolin 
from  Zettlitz,  37 ;  from  Western  N.  Caro- 
lina, 37  ;  weathered  shale,  60 ;  red-colored 
clay,  61  ;  of  clay  for  yellow  ware,  66  ;  flint 
clay,  68 ;  for  stoneware,  80 ;  Albany  slip 
clay,  83  ;  slip  clay,  86  ;  kaolin,  95,  96  ;  from 
Lawrence  Co.,  Ind.,  98 ;  from  Inyo  Co., 
Cal.,  98;  plastic  kaolin  from  Florida,  100 ; 
New  Jersey  clay,  101  ;  from  Jefferson  Co., 
Mo.,  and  Calloway  Co.,  Ky,  102  ;  Cornwall 

stone,  no,  112,  114;  from  Texas 115 

Clay,  importance  of  careful  sampling  for  analysis  of,  i ; 
necessity  for  careful  analysis  of,  3  ;  purification  of,  by 
washing  and  sifting,  3  ;  action  of  sulfuric  acid  upon..  9 
Clay,  physical  properties  of,  15  ;  coloring  of  burned,  15  ; 
physical  testing  of,  16  ;  mechanical  analysis  of,  17  ; 
plasticity  of,  18 ;  binding  power  of,  20  ;  burning  test 
for,  21  ;  for  floor  tile  and  paving  brick,  testing  for 

porosity,  24  ;  shrinkage 24 

Cobalt  oxid,  composition  of  commercial 132 

Copper  oxid,  use  as  coloring  material • 133 

Cornish  stone 118,119,120 

"  Crazing,"  48  ;  cause  of 5° 

Cream  colored  ware,  117,  glazes  for 122 

DISTILLED  water,  impurities  in 6 

Doulton  ware • 78 

"Bunting" 21 

Dust  pressed  articles 25 

ENAMEL,  description  of  the  term,  46  ;  see  also  glazes 
Enameled  brick,  139 ;  separation  of  glaze  from,  by  freezing 

and  thawing *47 

Enameled  tile,  127  ;  glazes  for 129 

Engobe  ware,  65  ;  for  white  enameled  brick 142,  145 

JFAIENCE 45.  62>  66 


INDEX.  195 

Fayal  pottery 43 

Feldspar,  analysis  of  Rorstrand,  37  ;  analysis  of,  from  N.  Y., 
37 ;    analyses  of  commercial,    108,    109 ;    Cornwall 

stone,  no,  112,  114;  from  Texas 114 

Ferric  oxid,   use  as  a  coloring  material,    136;    analysis   of 

' '  Crocus  Martis, ' ' 136 

Fire  brick 158 

Flint,  [see  quartz] 

Flint  clays,  analysis  of 170 

Flint  glass,  analysis  of 52 

Frit  melting 125 

GL,AZES.  Preparations  of  samples  for  analysis,  13 ;  con- 
taining boric  acid,  14  ;  necessary  qualities  of, 
48 ;  crazing  and  shivering,  50  ;  action  of  boric 
acid,  50;  silica,  51  ;  alumina,  51 ;  formula  for, 
51  ;  distinction  between  "raw  "  and  "fritted," 
53 ;  example  of  calculation  of,  54  ;  action  of 
sulfates  in,  55  ;  for  red  ware,  58 ;  for  Rockwood 
pottery,  62  ;  for  yellow  ware,  71  ;  for  Rocking- 
hatn  ware,  72  ;  salt  glaze,  81 ;  slip  glaze  for 
stoneware,  83,  91  ;  for  white  granite  and  C.  C. 
ware,  122  ;  for  majolica  and  enameled  tile,  129; 
for  white  enameled  brick,  141,  145,  146  ;  action 
of  frost  upon  glaze  for  enameled  brick 147 

Granite  ware 117 

HEINTZ'S  glass  mixtures 32 

' '  Hotel  china  " 44 

KJL/NS,   for   testing  clay,    23 ;    dimensions   of,   28 ;    down 

draught,  183  ;  [see  also  burning] 
"  LIMOGES"  ware..... 73 

TWAJOLJCA,  127;  glaz.es  for 129 

Majolica  and  faience,  distinction  between 45 

Manganese,  use  as  a  coloring  material 134 

Marble,  analysis  of  Carrara 37 


196  INDEX. 

Mica,  in  clay,  3  ;  clay  containing  products  of  the  decompo- 
sition of IT 

INICKElv  oxid,  use  and  composition 133 

PENNSYLVANIA  Dutch  pottery 62,  65 

' '  Plastic  kaolins  " 100 

Porosity  of  floor  tile  and  paving  brick,  test  for 24 

Prinsep,  alloys  of 30 

Pyrometry 26 

C^UARTZ,  in  clay,  9  ;  analysis  of  Norwegian,  37  ;  analysis 
of  Illinois,  38  ;  from  N.  J.  and  111.  analysis,  105  ; 

from  Tenn.,  106  ;  from  Ky 107 

'*  RED  ware,"  43,  58  ;  glaze  for,  58  ;  articles  made  of,  61 ; 

examination  of  materials  for 64 

Refractory  materials 158 

"  Rockingham  "  ware,  66 ;  glaze  for,   72;  articles  made  of, 

73  ;  examinations  of  materials  for 73 

Rookwood  pottery 62 

SAGGARS,  clay  for 163,  165 

Sampling  of  clay,  directions  for 2 

Sand,  [see  quartz] 

Seger  &  Aron's  rational  analysis  of  clay 8 

Seger's  cones,  33  ;   mixtures  for  preparing,  38 ;  use  of,  40 ; 

composition  and  use  of 173 

"  Shivering,"  48 ;  cause  of 50 

Shrinkage  in  firing,  test  of 24 

Silica,  distinction  between  quartz  and  combined,  9 ;  separa- 
tion from  alumina  and  ferric  oxid 4,  5 

"Slips" 65 

Stoneware,  77 ;  articles  made  of,  77  ;  use  of,  in  chemical 
industries,  78  :  clay  for,  79  ;  glazes,  81 ;  exami- 
nation of  material 92 

Sulfates  objectionable  in  glazes,  55  ;  in  floor  tile  and  terra 

cotta,  153  ;  action  in  the  kiln 189 

TEMPERATURE  of  the  kiln,  methods  for  determining. . .     26 


INDEX.  197 

Terra-cotta,  149  ;  shrinkage  in  burning 152 

Thenard's  blue,  composition  of 120 

Tile,  enameled,    127;     glazes  for,    129;  floor,  149 ;     roofing, 

156  ;  encaustic 156 

Tin  oxid,  use  in  enamels 137 

UMBER,  use  as  a  coloring  material 134 

Uranium,  use  as  a  coloring  material 135 

VICAT'S  needle 18 

1A/ATER,  impurities  in  distilled 6 

"  W.  G."  ware 43 

White  granite  ware,  117  ;  glazes  for 122 

White  lead,  deleterious  influence  on  glazes  of  acetate  con- 
tained in 125 

White  ware 93 

Whiting,  analysis  of  38 ;  composition  of 116 

YELLOW  ware,  66  ;  preparation  of  the  clay,  67 ;  glaze  for, 
71  ;  articles  made  of,   73  ;  examination  of 

materials  for 73 


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